Scientific Research with the Space Telescope: International Astronomical Union Colloquium No. 54. [conferences

NASA Technical Reports Server (NTRS)

Longair, M. S.; Warner, J. W.

1979-01-01

The application of the space telescope for extragalactic astronomy, planetary research, and stellar, interstellar, and galactic structural problems is discussed. Topics include investigations of small solar system objects, the physical characteristics of ionized gaseous nebulae, the central regions of active galaxies and quasars, problems of cosmology, and the distribution and composition of interstellar matter.

Interstellar scintillation observations for PSR B0355+54

NASA Astrophysics Data System (ADS)

Xu, Y. H.; Lee, K. J.; Hao, L. F.; Wang, H. G.; Liu, Z. Y.; Yue, Y. L.; Yuan, J. P.; Li, Z. X.; Wang, M.; Dong, J.; Tan, J. J.; Chen, W.; Bai, J. M.

2018-06-01

In this paper, we report our investigation of pulsar scintillation phenomena by monitoring PSR B0355+54 at 2.25 GHz for three successive months using the Kunming 40-m radio telescope. We measured the dynamic spectrum, the two-dimensional correlation function and the secondary spectrum. These observations have a high signal-to-noise ratio (S/N ≥ 100). We detected scintillation arcs, which are rarely observable using such a small telescope. The sub-microsecond scale width of the scintillation arc indicates that the transverse scale of the structures on the scattering screen is as compact as astronomical unit size. Our monitoring shows that the scintillation bandwidth, the time-scale and the arc curvature of PSR B0355+54 were varying temporally. A plausible explanation would need to invoke a multiple-scattering-screen or multiple-scattering-structure scenario, in which different screens or ray paths dominate the scintillation process at different epochs.

FOREST unbiased Galactic plane imaging survey with the Nobeyama 45 m telescope (FUGIN). I. Project overview and initial results

NASA Astrophysics Data System (ADS)

Umemoto, Tomofumi; Minamidani, Tetsuhiro; Kuno, Nario; Fujita, Shinji; Matsuo, Mitsuhiro; Nishimura, Atsushi; Torii, Kazufumi; Tosaki, Tomoka; Kohno, Mikito; Kuriki, Mika; Tsuda, Yuya; Hirota, Akihiko; Ohashi, Satoshi; Yamagishi, Mitsuyoshi; Handa, Toshihiro; Nakanishi, Hiroyuki; Omodaka, Toshihiro; Koide, Nagito; Matsumoto, Naoko; Onishi, Toshikazu; Tokuda, Kazuki; Seta, Masumichi; Kobayashi, Yukinori; Tachihara, Kengo; Sano, Hidetoshi; Hattori, Yusuke; Onodera, Sachiko; Oasa, Yumiko; Kamegai, Kazuhisa; Tsuboi, Masato; Sofue, Yoshiaki; Higuchi, Aya E.; Chibueze, James O.; Mizuno, Norikazu; Honma, Mareki; Muller, Erik; Inoue, Tsuyoshi; Morokuma-Matsui, Kana; Shinnaga, Hiroko; Ozawa, Takeaki; Takahashi, Ryo; Yoshiike, Satoshi; Costes, Jean; Kuwahara, Sho

2017-10-01

The FUGIN project is one of legacy projects using a new multi-beam FOREST (four-beam receiver system on the 45 m telescope). This project aims to simultaneously investigate the distribution, kinematics, and physical properties of both diffuse and dense molecular gases in the Galaxy by observing 12CO, 13CO, and C18O J = 1-0 lines simultaneously. Mapping regions are parts of the first quadrant (10° ≤ l ≤ 50°, |b| ≤ 1°) and the third quadrant (198° ≤ l ≤ 236°, |b| ≤ 1°) of the Galaxy, where spiral arms, bar structure, and the molecular gas ring are included. This survey achieves the highest angular resolution to date (˜20″) for the Galactic plane survey in the CO J = 1-0 lines, which makes it possible to find dense clumps located farther away than the previous surveys. FUGIN will provide us an invaluable dataset for investigating the physics of the Galactic interstellar medium (ISM), particularly the evolution of interstellar gas covering galactic-scale structures to the internal structures of giant molecular clouds, such as small filaments/clumps/cores. We present an overview of the FUGIN project, the observation plan and initial results. These results reveal wide-field and detailed structures of molecular clouds, such as entangled filaments that have not been obvious in previous surveys, and large-scale kinematics of molecular gas, such as spiral arms.

The influence of cosmic rays on the stability and large-scale dynamics of the interstellar medium

NASA Astrophysics Data System (ADS)

Kuznetsov, V. D.

1986-06-01

The diffusion-convection formulation is used to study the influence of galactic cosmic rays on the stability and dynamics of the interstellar medium which is supposedly kept in equilibrium by the gravitational field of stars. It is shown that the influence of cosmic rays on the growth rate of MHD instability depends largely on a dimensionless parameter expressing the ratio of the characteristic acoustic time scale to the cosmic-ray diffusion time. If this parameter is small, the cosmic rays will decelerate the build-up of instabilities, thereby stabilizing the system; in contrast, if the parameter is large, the system will be destabilized.

The Chemical Composition and Gas-to-Dust Mass Ratio of Nearby Interstellar Matter

NASA Technical Reports Server (NTRS)

Frisch, Priscilla C.; Slavin, Jonathan D.

2003-01-01

We use recent results on interstellar gas toward nearby stars and interstellar by-products within the solar system to select among the equilibrium radiative transfer models of the nearest interstellar material presented in Slavin & Frisch. For the assumption that O/H - 400 parts per million, models 2 and 8 are found to yield good fits to available data on interstellar material inside and outside of the heliosphere, with the exception of the Ne abundance in the pickup ion and anomalous cosmic-ray populations. For these models, the interstellar medium (ISM) at the entry point to the heliosphere has n(H(sup 0)) = 0.202-0.208/cu cm, n(He(sup 0) = 0.0137-0.0152/cu cm, and ionizations X(H) = 0.29-0.30, X(He) = 0.47-0.51. These best models suggest that the chemical composition of the nearby ISM is approx.60%-70% subsolar if S is undepleted. Both H(0) and H(+) need to be included when evaluating abundances of ions found in warm diffuse clouds. Models 2 and 8 yield an H filtration factor of approx.0.46. Gas-to-dust mass ratios for the ISM toward epsilon CMa are R(sub gd) = 178-183 for solar abundances of Holweger or R(sub gd) = 611-657 for an interstellar abundance standard 70% solar. Direct observations of dust grains in the solar system by Ulysses and Galileo yield R(sub gd) appr0x. 115 for models 2 and 8, supporting earlier results (Frisch and coworkers). If the local ISM abundances are subsolar, then gas and dust are decoupled over small spatial scales. The inferred variation in R(sub gd) over parsec length scales is consistent with the fact that the ISM near the Sun is part of a dynamically active cluster of cloudlets flowing away from the Sco-Cen association. Observations toward stars within approx.500 pc show that R(sub gd) correlates with the percentage of the dust mass that is carried by iron, suggesting that an Fe-rich grain core (by mass) remains after grain destruction. Evidently large dust grains (>10(exp -13) g) and small dust grains (<10(exp -13) g) are not well mixed over parsec length spatial scales in the ISM. It also appears that very small C-dominated dust grains have been destroyed in the ISM within several parsecs of the Sun, since C appears to be essentially undepleted. However, if gas-dust coupling breaks down over the cloud lifetime, the missing mass arguments applied here to determine R(sub gd) and dust grain mineralogy are not appropriate.

Enhanced Resolution Maps of Energetic Neutral Atoms from IBEX

NASA Astrophysics Data System (ADS)

Teodoro, L. A.; Elphic, R. C.; Janzen, P.; Reisenfeld, D.; Wilson, J. T.

2017-12-01

The discovery by the Interstellar Boundary Explorer (IBEX) of a "Ribbon" in the measurements of Energetic Neutral Particles (ENA) was a major surprise that lead to the re-thinking of the Physics underpinning the heliosphere-intergalactic medium boundary dynamics. Several physical models have been proposed and tested in their ability to mimic the IBEX observations. Some of the ENA IBEX's include the following features: 1) The presence of fine structure within the ribbon suggests that the physical properties of it exhibit small-scale spacial structure and possibly rapid small-scale variations. 2) The ribbon is a fairly narrow feature at low energies and broadens with increasing energy;The IBEX detectors were designed to maximize count rate by incorporating wide angular and broad energy acceptance. Thus far, the existing mapping software used by the IBEX Science Operation Center has not been design with the "Ribbon" ( 20o wide) in mind: the current generation of maps are binned in 6o longitude pixels by 6o latitude pixels (so the pixels are all of the same size in angle and are quite "blocky"). Furthermore, the instrumental point spread function has not been deconvolved, making any potential narrow features broader than they are. An improvement in the spatial resolution of the IBEX maps would foster a better understanding of the Ribbon and its substructure, and thus reply to some of the basic and profound questions related to its origin, the nature of the outer boundaries of the our solar system and the surrounding interstellar Galactic medium.Here we report on the application of the Bayesian image reconstruction algorithm "Speedy Pixons" to the ENA data with the aim to sharpen the ENA IBEX maps. A preliminary application allow us to conclude that: The peaks in the count rate do appear to be more enhanced in the reconstruction; The reconstruction is clearly denoised; The "Ribbon" is better defined in the reconstruction. We are currently studying the implications of our preliminary results in the current generation of models. Potentially, our results can also be used in the design and planning of future missions whose aim is to produce higher resolution maps of the interstellar medium (e.g. IMAP).

Molecular gas excitation in Kleinmann-Low nebula

NASA Astrophysics Data System (ADS)

Callejo, Gonzague; Lemaire, Jean-Louis; Le Petit, F.; Pineau Des Forets, Guillaume; Field, David

2004-12-01

The Orion molecular cloud OMC-1 is the perfect object for the study of the dense interstellar medium accounting for active star formation; the intrinsic complexity of this region serves as a stallion, both for observing techniques and for interstellar medium modelling. A detailed dynamic and spectroscopic study has been performed using VLT and CFHT observations of the infrared Kleinmann-Low nebula; yielding a complete small-scale structure in velocity and most importantly powerful diagnosis tools in order to put a new light into the gas behaviour. These results allow to build up a consistent model of the gas excitation, and a clear dynamical view of the region. The crucial action of the shock waves is confirmed, and the discrepancies between the observations and the standard models are discussed. The consequences of this modelling will be discussed in terms of extinction, magnetic field, and other quantities related to induced star formation. The goodness of the models used for the interpretations will be also discussed and some future directions of investigation enhanced.

Recombination Rates of Electrons with Interstellar PAH Molecules

NASA Technical Reports Server (NTRS)

Ballester, Jorge (Cartographer)

1996-01-01

The goal of this project is to develop a general model for the recombination of electrons with PAH molecules in an interstellar environment. The model is being developed such that it can be applied to a small number of families of PAHs without reference to specific molecular structures. Special attention will be focused on modeling the approximately circular compact PAHs in a way that only depends on the number of carbon atoms.

Electron energy loss spectrometry of interstellar diamonds

NASA Technical Reports Server (NTRS)

Bernatowicz, Thomas J.; Gibbons, Patrick C.; Lewis, Roy S.

1990-01-01

The results are reported of electron energy loss spectra (EELS) measurements on diamond residues from carbonaceous meteorites designed to elucidate the structure and composition of interstellar diamonds. Dynamic effective medium theory is used to model the dielectric properties of the diamonds and in particular to synthesize the observed spectra as mixtures of diamond and various pi-bonded carbons. The results are shown to be quantitatively consistent with the idea that diamonds and their surfaces are the only contributors to the electron energy loss spectra of the diamond residues and that these peculiar spectra are the result of the exceptionally small grain size and large specific surface area of the interstellar diamonds.

The impact of different interstellar medium structures on the dynamical evolution of supernova remnants

NASA Astrophysics Data System (ADS)

Wang, Yueyang; Bao, Biwen; Yang, Chuyuan; Zhang, Li

2018-05-01

The dynamical properties of supernova remnants (SNRs) evolving with different interstellar medium structures are investigated through performing extensive two-dimensional magnetohydrodynamic (MHD) simulations in the cylindrical symmetry. Three cases of different interstellar medium structures are considered: the uniform medium, the turbulent medium and the cloudy medium. Large-scale density and magnetic fluctuations are calculated and mapped into the computational domain before simulations. The clouds are set by random distribution in advance. The above configuration allows us to study the time-dependent dynamical properties and morphological evolution of the SNR evolving with different ambient structures, along with the development of the instabilities at the contact discontinuity. Our simulation results indicate that remnant morphology deviates from symmetry if the interstellar medium contains clouds or turbulent density fluctuations. In the cloudy medium case, interactions between the shock wave and clouds lead to clouds' fragmentation. The magnetic field can be greatly enhanced by stretching field lines with a combination of instabilities while the width of amplification region is quite different among the three cases. Moreover, both the width of amplification region and the maximum magnetic-field strength are closely related to the clouds' density.

Fine-scale structure in the far-infrared Milky-Way

NASA Technical Reports Server (NTRS)

Waller, William H.; Wall, William F.; Reach, William T.; Varosi, Frank; Ebert, Rick; Laughlin, Gaylin; Boulanger, Francois

1995-01-01

This final report summarizes the work performed and which falls into five broad categories: (1) generation of a new data product (mosaics of the far-infrared emission in the Milky Way); (2) acquisition of associated data products at other wavelengths; (3) spatial filtering of the far-infrared mosaics and resulting images of the FIR fine-scale structure; (4) evaluation of the spatially filtered data; (5) characterization of the FIR fine-scale structure in terms of its spatial statistics; and (6) identification of interstellar counterparts to the FIR fine-scale structure.

Very Small Interstellar Spacecraft

NASA Astrophysics Data System (ADS)

Peck, Mason A.

2007-02-01

This paper considers lower limits of length scale in spacecraft: interstellar vehicles consisting of little more material than found in a typical integrated-circuit chip. Some fundamental scaling principles are introduced to show how the dynamics of the very small can be used to realize interstellar travel with minimal advancements in technology. Our recent study for the NASA Institute for Advanced Concepts provides an example: the use of the Lorentz force that acts on electrically charged spacecraft traveling through planetary and stellar magnetospheres. Schaffer and Burns, among others, have used Cassini and Voyager imagery to show that this interaction is responsible for some of the resonances in the orbital dynamics of dust in Jupiter's and Saturn's rings. The Lorentz force turns out to vary in inverse proportion to the square of this characteristic length scale, making it a more effective means of propelling tiny spacecraft than solar sailing. Performance estimates, some insight into plasma interactions, and some hardware concepts are offered. The mission architectures considered here involve the use of these propellantless propulsion techniques for acceleration within our solar system and deceleration near the destination. Performance estimates, some insight into plasma interactions, and some hardware concepts are offered. The mission architectures considered here involve the use of these propellantless propulsion techniques for acceleration within our solar system and deceleration near the destination. We might envision a large number of such satellites with intermittent, bursty communications set up as a one-dimensional network to relay signals across great distances using only the power likely from such small spacecraft. Conveying imagery in this fashion may require a long time because of limited power, but the prospect of imaging another star system close-up ought to be worth the wait.

UV-optical from space

NASA Technical Reports Server (NTRS)

Illingworth, Garth; Savage, Blair; Angel, J. Roger; Blandford, Roger D.; Boggess, Albert; Bowyer, C. Stuart; Carruthers, George R.; Cowie, Lennox L.; Doschek, George A.; Dupree, Andrea K.

1991-01-01

The following subject areas are covered: (1) the science program (star formation and origins of planetary systems; structure and evolution of the interstellar medium; stellar population; the galactic and extragalactic distance scale; nature of galaxy nuclei, AGNs, and QSOs; formation and evolution of galaxies at high redshifts; and cosmology); (2) implementation of the science program; (3) the observatory-class missions (HST; LST - the 6m successor to HST; and next-generation 16m telescope); (4) moderate and small missions (Delta-class Explorers; imaging astrometric interferometer; small Explorers; optics development and demonstrations; and supporting ground-based capabilities); (5) prerequisites - the current science program (Lyman-FUSE; HTS optimization; the near-term science program; data analysis, modeling, and theory funding; and archives); (6) technologies for the next century; and (7) lunar-based telescopes and instruments.

On Galactic Density Modeling in the Presence of Dust Extinction

NASA Astrophysics Data System (ADS)

Bovy, Jo; Rix, Hans-Walter; Green, Gregory M.; Schlafly, Edward F.; Finkbeiner, Douglas P.

2016-02-01

Inferences about the spatial density or phase-space structure of stellar populations in the Milky Way require a precise determination of the effective survey volume. The volume observed by surveys such as Gaia or near-infrared spectroscopic surveys, which have good coverage of the Galactic midplane region, is highly complex because of the abundant small-scale structure in the three-dimensional interstellar dust extinction. We introduce a novel framework for analyzing the importance of small-scale structure in the extinction. This formalism demonstrates that the spatially complex effect of extinction on the selection function of a pencil-beam or contiguous sky survey is equivalent to a low-pass filtering of the extinction-affected selection function with the smooth density field. We find that the angular resolution of current 3D extinction maps is sufficient for analyzing Gaia sub-samples of millions of stars. However, the current distance resolution is inadequate and needs to be improved by an order of magnitude, especially in the inner Galaxy. We also present a practical and efficient method for properly taking the effect of extinction into account in analyses of Galactic structure through an effective selection function. We illustrate its use with the selection function of red-clump stars in APOGEE using and comparing a variety of current 3D extinction maps.

Structure of small-scale magnetic fields in the kinematic dynamo theory.

PubMed

Schekochihin, Alexander; Cowley, Steven; Maron, Jason; Malyshkin, Leonid

2002-01-01

A weak fluctuating magnetic field embedded into a a turbulent conducting medium grows exponentially while its characteristic scale decays. In the interstellar medium and protogalactic plasmas, the magnetic Prandtl number is very large, so a broad spectrum of growing magnetic fluctuations is excited at small (subviscous) scales. The condition for the onset of nonlinear back reaction depends on the structure of the field lines. We study the statistical correlations that are set up in the field pattern and show that the magnetic-field lines possess a folding structure, where most of the scale decrease is due to the field variation across itself (rapid transverse direction reversals), while the scale of the field variation along itself stays approximately constant. Specifically, we find that, though both the magnetic energy and the mean-square curvature of the field lines grow exponentially, the field strength and the field-line curvature are anticorrelated, i.e., the curved field is relatively weak, while the growing field is relatively flat. The detailed analysis of the statistics of the curvature shows that it possesses a stationary limiting distribution with the bulk located at the values of curvature comparable to the characteristic wave number of the velocity field and a power tail extending to large values of curvature where it is eventually cut off by the resistive regularization. The regions of large curvature, therefore, occupy only a small fraction of the total volume of the system. Our theoretical results are corroborated by direct numerical simulations. The implication of the folding effect is that the advent of the Lorentz back reaction occurs when the magnetic energy approaches that of the smallest turbulent eddies. Our results also directly apply to the problem of statistical geometry of the material lines in a random flow.

Synthetic Observations of 21 cm H I Line Profiles from Inhomogeneous Turbulent Interstellar H I Gas with Magnetic Fields

NASA Astrophysics Data System (ADS)

Fukui, Yasuo; Hayakawa, Takahiro; Inoue, Tsuyoshi; Torii, Kazufumi; Okamoto, Ryuji; Tachihara, Kengo; Onishi, Toshikazu; Hayashi, Katsuhiro

2018-06-01

We carried out synthetic observations of interstellar atomic hydrogen at 21 cm wavelength by utilizing the magnetohydrodynamic numerical simulations of the inhomogeneous turbulent interstellar medium. The cold neutral medium (CNM) shows a significantly clumpy distribution with a small volume filling factor of 3.5%, whereas the warm neutral medium (WNM) has a distinctly different and smooth distribution with a large filling factor of 96.5%. In projection on the sky, the CNM exhibits a highly filamentary distribution with a subparsec width, whereas the WNM shows a smooth, extended distribution. In the H I optical depth, the CNM is dominant and the contribution of the WNM is negligibly small. The CNM has an area covering factor of 30% in projection, while the WNM has a covering factor of 70%. This means that the emission–absorption measurements toward radio continuum compact sources tend to sample the WNM with a probability of 70%, yielding a smaller H I optical depth and a smaller H I column density than those of the bulk H I gas. The emission–absorption measurements, which are significantly affected by the small-scale large fluctuations of the CNM properties, are not suitable for characterizing the bulk H I gas. Larger-beam emission measurements that are able to fully sample the H I gas will provide a better tool for that purpose, if a reliable proxy for hydrogen column density, possibly dust optical depth and gamma rays, is available. The present results provide a step toward precise measurements of the interstellar hydrogen with ∼10% accuracy. This will be crucial in interstellar physics, including identification of the proton–proton interaction in gamma-ray supernova remnants.

Silicon chemistry in interstellar clouds

NASA Technical Reports Server (NTRS)

Langer, William D.; Glassgold, A. E.

1990-01-01

A new model of interstellar silicon chemistry is presented that explains the lack of SiO detections in cold clouds and contains an exponential temperature dependence for the SiO abundance. A key aspect of the model is the sensitivity of SiO production by neutral silicon reactions to density and temperature, which arises from the dependence of the rate coefficients on the population of the excited fine-structure levels of the silicon atom. As part of the explanation of the lack of SiO detections at low temperatures and densities, the model also emphasizes the small efficiencies of the production routes and the correspondingly long times needed to reach equilibrium. Measurements of the abundance of SiO, in conjunction with theory, can provide information on the physical properties of interstellar clouds such as the abundance of oxygen bearing molecules and the depletion of interstellar silicon.

Historical Reveiw of Interstellar Probe Concepts and Examination of Payload Mass Considerations for Different System Architectures

NASA Astrophysics Data System (ADS)

Long, K.

2017-12-01

The ability to send a space probe beyond the Voyager probes, through the interstellar medium and towardsthe distant stars, has long been the ambition of both the science ction literature but also a small community ofadvocates that have argued for a broader and deeper vision of space exploration that goes outside of our SolarSystem. In this paper we discuss some of the historical interstellar probe concepts which are propelled usingdierent types of propulsion technology, from energetic reaction engines to directed energy beaming, and considerthe payload mass associated with such concepts. We compare and contrast the dierent design concepts, payloadmass fractions, powers and energies and discuss the implications for robotic space exploration within the stellarneighbourhood. Finally, we consider the Breakthrough Starshot initiative, which proposes to send a Gram-scalelaser driven spacecraft to the Alpha Centauri system in a 20 year mission travelling at v 0.2c. We show howthis is a good start in pushing our robotic probes towards interstellar destinations, but also discuss the potentialfor scaling up this systems architecture to missions closer at home, or higher mass missions wider aeld. This is apresentation for the American Geophysical Union at the AGU Fall meeting, New Orleans, 11-15 December 2017,Special Session on the Interstellar Probe Missions.Keywords: Interstellar Probe, Breakthrough Starshot

The Propagation Distance and Sources of Interstellar Turbulence

NASA Astrophysics Data System (ADS)

Spangler, S. R.

2007-07-01

Turbulence appears to be widely distributed in the interstellar medium, including regions far from obvious generators of this turbulence such as supernova remnants and star formation regions. This indicates that the turbulence must be transported, most likely by propagation at the Alfvén speed, over distances of hundreds of parsecs. This requirement appears contradicted by estimates that the damping length of magnetohydrodynamic waves and turbulence by ion-neutral collisions in the Diffuse Ionized Gas (DIG, the most pervasive phase of the interstellar medium) is less than a parsec. This damping length estimate is not highly model-dependent, and is consistent with calculations positing a balance between radiative cooling and turbulent dissipative heating of the interstellar gas. This problem is even more severe in the Warm Neutral Medium (WNM) phase, where the neutral density fraction is much higher. Three possible resolutions of this matter are proposed. (1) Interstellar turbulence may be generated by highly distributed, local generators rather than greatly separated, powerful generators such as supernova remnants. (2) The turbulence may be generated by powerful and isolated objects like supernova remnants, but then ``percolate'' through the interstellar medium by propagating through channels with a very high degree of ionization. (3) The dissipation of small-scale turbulence may be balanced by a cascade from larger, less damped fluctuations.

Shock Structure: Application to the heliospheric termination shock and an interstellar shock

NASA Astrophysics Data System (ADS)

Mostafavi, P.; Zank, G. P.

2017-12-01

The structure of parallel and perpendicular shocks is often mediated by energetic particles. Here we describe shock structure when mediated by energetic particle heat flux and viscosity. We present a general theoretical model of shock mediation, which is then applied to Voyager 2 observations of the heliospheric termination shock (HTS) and Voyage 1 observations of a shock in very local interstellar medium (VLISM). Voyager 2 observations showed that the downstream HTS flow remained supersonic with respect to the thermal gas [Richardson et al., 2008]. Thus the thermal gas remains cold through the HTS and does not provide the dissipation to account for the deceleration of the supersonic solar wind. We show that PUIs are the primary dissipation mechanism and gain most of the solar wind kinetic energy in crossing the HTS. The interstellar shock observed by Voyager 1 [Burlaga et al., 2013] was extremely broad and so far there no theoretical explanation has been provided that describes the VLISM shock structure. Using the Chandrasekhar function, we show that the VLISM is collisional with respect to the thermal plasma and that electron and proton collisional mean free paths are very small. Thus, thermal collisionality should determine the structure of VLISM shocks. PUIs outside the heliosphere are generated by secondary charge exchange and contribute a very small pressure. Since PUIs and the dissipation associated with them cannot mediate the shock observed in the VLISM, we suggest that the thickness of the shock observed in the VLISM is due to collisional thermal gas dissipation.

A Heliosphere Buffeted by Interstellar Turbulence?

NASA Astrophysics Data System (ADS)

Jokipii, J. R.; Giacalone, J.

2014-12-01

Recent observations from IBEX combined with previous measurements from other sources suggest new, local, effects of interstellar turbulence. Observations of various interstellar parameters such as the magnetic field, fluid velocity and electron density, over large spatial scales, have revealed a broadband Kolmogorov spectrum of interstellar turbulence which pervades most of interstellar space. The outer scale (or coherence scale of this turbulence) is found to be approximately 10^19 cm and the inner cutoff scale is less than 1000 km. The root-mean-square relative fluctuation in the fluid and the magnetic-field parameters is of order unity. If this turbulence exists at the heliosphere, the root-mean-square relative fluctuations at 100 (heliospheric) AU scales is approximately 0.1. The recently published value for the change In observed velocity direction for the interstellar flow relative to the heliosphere (Frisch, etal, 2014)is consistent with this. Similarly, interpreting the width of the IBEX ribbon in terms of a fluctuating magnetic field also is in agreement with this picture. Observations of TeV cosmic rays can also be explained. Potential effects of these fluctuations in the interstellar medium on the heliosphere will be discussed. Reference: Frisch, etal, Science, 341, 480

Varying the forcing scale in low Prandtl number dynamos

NASA Astrophysics Data System (ADS)

Brandenburg, A.; Haugen, N. E. L.; Li, Xiang-Yu; Subramanian, K.

2018-06-01

Small-scale dynamos are expected to operate in all astrophysical fluids that are turbulent and electrically conducting, for example the interstellar medium, stellar interiors, and accretion disks, where they may also be affected by or competing with large-scale dynamos. However, the possibility of small-scale dynamos being excited at small and intermediate ratios of viscosity to magnetic diffusivity (the magnetic Prandtl number) has been debated, and the possibility of them depending on the large-scale forcing wavenumber has been raised. Here we show, using four values of the forcing wavenumber, that the small-scale dynamo does not depend on the scale-separation between the size of the simulation domain and the integral scale of the turbulence, i.e., the forcing scale. Moreover, the spectral bottleneck in turbulence, which has been implied as being responsible for raising the excitation conditions of small-scale dynamos, is found to be invariant under changing the forcing wavenumber. However, when forcing at the lowest few wavenumbers, the effective forcing wavenumber that enters in the definition of the magnetic Reynolds number is found to be about twice the minimum wavenumber of the domain. Our work is relevant to future studies of small-scale dynamos, of which several applications are being discussed.

Fine Structure of Anomalously Intense Pulses of PSR J0814+7429 Radio Emission in the Decameter Range

NASA Astrophysics Data System (ADS)

Skoryk, A. O.; Ulyanov, O. M.; Zakharenko, V. V.; Shevtsova, A. I.; Vasylieva, I. Y.; Plakhov, M. S.; Kravtsov, I. M.

2017-06-01

Purpose: The fine structure of the anomalously intense pulses of PSR J0814+7429 (B0809+74) has been studied. The pulsar radio emission fine structure is investigated to determine its parameters in the lowest part of spectrum available for groundbased observations. Design/methodology/approach: The scattering measure in the interstellar plasma have been estimated using the spectral and correlation analyses of pulsar data recorded by the UTR-2 radio telescope. Results: Two characteristic time scales of the anomalously intense pulses fine structure of the PSR J0814+7429 radio emission have been found. The strongest pulses of this pulsar in the decameter range can have a duration of about t 2÷3 ms. These pulses are emitted in short series. In some cases, they are emitted over the low-intensity plateau consisting of the “long” subpulse component. Conclusions: The narrowest correlation scale of pulsar J0814+7429 radio emission corresponds to the doubled scattering time constant of the interstellar medium impulse response. Broader scale of the fine structure of its radio emission can be explained by the radiation of a short series of narrow pulses or relatively broad pulses inside this pulsar magnetosphere.

Spectral Properties of Gas-phase Condensed Fullerene-like Carbon Nanoparticles from Far-ultraviolet to Infrared Wavelengths

NASA Astrophysics Data System (ADS)

Jäger, C.; Mutschke, H.; Henning, Th.; Huisken, F.

2008-12-01

Carbon solids are ubiquitous material in interstellar space. However, the formation pathway of carbonaceous matter in astrophysical environments, as well as in terrestrial gas-phase condensation reactions, is not yet understood. Laser ablation of graphite in different quenching gas atmospheres, such as pure He, He/H2, and He/H2O at varying pressures, is used to synthesize very small, fullerene-like carbon nanoparticles. The particles are characterized by very small diameters between 1 and 4 nm and a disturbed onion-like structure. The soot particles extracted from the condensation zone obviously represent a very early stage of particle condensation. The spectral properties have been measured from the far-ultraviolet (FUV; λ = 120 nm) to the mid-infrared (MIR; λ = 15 μm). The seedlike soot particles show strong absorption bands in the 3.4 μm range. The profile and the intensity pattern of the 3.4 μm band of the diffuse interstellar medium can be well reproduced by the measured 3.4 μm profile of the condensed particles; however, all the carbon which is left to form solids is needed to fit the intensity of the interstellar bands. In contrast to the assumption that onion-like soot particles could be the carriers of the interstellar ultraviolet (UV) bump, our very small onion-like carbon nanoparticles do not show distinct UV bands due to (π-π*) transitions.

Non-monotonic spatial distribution of the interstellar dust in astrospheres: finite gyroradius effect

NASA Astrophysics Data System (ADS)

Katushkina, O. A.; Alexashov, D. B.; Izmodenov, V. V.; Gvaramadze, V. V.

2017-02-01

High-resolution mid-infrared observations of astrospheres show that many of them have filamentary (cirrus-like) structure. Using numerical models of dust dynamics in astrospheres, we suggest that their filamentary structure might be related to specific spatial distribution of the interstellar dust around the stars, caused by a gyrorotation of charged dust grains in the interstellar magnetic field. Our numerical model describes the dust dynamics in astrospheres under an influence of the Lorentz force and assumption of a constant dust charge. Calculations are performed for the dust grains with different sizes separately. It is shown that non-monotonic spatial dust distribution (viewed as filaments) appears for dust grains with the period of gyromotion comparable with the characteristic time-scale of the dust motion in the astrosphere. Numerical modelling demonstrates that the number of filaments depends on charge-to-mass ratio of dust.

STRUCTURE OF THE INTERSTELLAR BOUNDARY EXPLORER RIBBON FROM SECONDARY CHARGE-EXCHANGE AT THE SOLAR–INTERSTELLAR INTERFACE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zirnstein, E. J.; McComas, D. J.; Heerikhuisen, J., E-mail: ezirnstein@swri.edu, E-mail: dmccomas@swri.edu, E-mail: jacob.heerikhuisen@uah.edu

2015-05-01

In 2009, the Interstellar Boundary Explorer discovered a bright “ribbon” of energetic neutral atom (ENA) flux in the energy range ≤0.4–6 keV, encircling a large portion of the sky. This observation was not previously predicted by any models or theories, and since its discovery, it has been the subject of numerous studies of its origin and properties. One of the most studied mechanisms for its creation is the “secondary ENA” process. Here, solar wind ions, neutralized by charge-exchange with interstellar atoms, propagate outside the heliopause; experience two charge-exchange events in the dense outer heliosheath; and then propagate back inside themore » heliosphere, preferentially in the direction perpendicular to the local interstellar magnetic field. This process has been extensively analyzed using state-of-the-art modeling and simulation techniques, but it has been difficult to visualize. In this Letter, we show the three-dimensional structure of the source of the ribbon, providing a physical picture of the spatial and energy scales over which the secondary ENA process occurs. These results help us understand how the ribbon is generated and further supports a secondary ENA process as the leading ribbon source mechanism.« less

INTERPRETATION OF THE STRUCTURE FUNCTION OF ROTATION MEASURE IN THE INTERSTELLAR MEDIUM

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xu, Siyao; Zhang, Bing, E-mail: syxu@pku.edu.cn, E-mail: zhang@physics.unlv.edu

2016-06-20

The observed structure function (SF) of rotation measure (RM) varies as a broken power-law function of angular scales. The systematic shallowness of its spectral slope is inconsistent with the standard Kolmogorov scaling. This motivates us to examine the statistical analysis on RM fluctuations. The correlations of RM constructed by Lazarian and Pogosyan are demonstrated to be adequate in explaining the observed features of RM SFs through a direct comparison between the theoretically obtained and observationally measured SF results. By segregating the density and magnetic field fluctuations and adopting arbitrary indices for their respective power spectra, we find that when themore » SFs of RM and emission measure have a similar form over the same range of angular scales, the statistics of the RM fluctuations reflect the properties of density fluctuations. RM SFs can be used to evaluate the mean magnetic field along the line of sight, but cannot serve as an informative source on the properties of turbulent magnetic field in the interstellar medium. We identify the spectral break of RM SFs as the inner scale of a shallow spectrum of electron density fluctuations, which characterizes the typical size of discrete electron density structures in the observed region.« less

Is interstellar archeology possible?

NASA Astrophysics Data System (ADS)

Carrigan, Richard A.

2012-09-01

Searching for signatures of cosmic-scale archeological artifacts such as Dyson spheres is an interesting alternative to conventional radio SETI. Uncovering such an artifact does not require the intentional transmission of a signal on the part of the original civilization. This type of search is called interstellar archeology or sometimes cosmic archeology. A variety of interstellar archeology signatures is discussed including non-natural planetary atmospheric constituents, stellar doping, Dyson spheres, as well as signatures of stellar, and galactic-scale engineering. The concept of a Fermi bubble due to interstellar migration is reviewed in the discussion of galactic signatures. These potential interstellar archeological signatures are classified using the Kardashev scale. A modified Drake equation is introduced. With few exceptions interstellar archeological signatures are clouded and beyond current technological capabilities. However SETI for so-called cultural transmissions and planetary atmosphere signatures are within reach.

Discovery of pulsed OH maser emission stimulated by a pulsar.

PubMed

Weisberg, Joel M; Johnston, Simon; Koribalski, Bärbel; Stanimirovic, Snezana

2005-07-01

Stimulated emission of radiation has not been directly observed in astrophysical situations up to this time. Here we demonstrate that photons from pulsar B1641-45 stimulate pulses of excess 1720-megahertz line emission in an interstellar hydroxyl (OH) cloud. As this stimulated emission is driven by the pulsar, it varies on a few-millisecond time scale, which is orders of magnitude shorter than the quickest OH maser variations previously detected. Our 1612-megahertz spectra are inverted copies of the 1720-megahertz spectra. This "conjugate line" phenomenon enables us to constrain the properties of the interstellar OH line-producing gas. We also show that pulsar signals undergo significantly deeper OH absorption than do other background sources, which confirms earlier tentative findings that OH clouds are clumpier on small scales than are neutral hydrogen clouds.

ON GALACTIC DENSITY MODELING IN THE PRESENCE OF DUST EXTINCTION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bovy, Jo; Rix, Hans-Walter; Schlafly, Edward F.

Inferences about the spatial density or phase-space structure of stellar populations in the Milky Way require a precise determination of the effective survey volume. The volume observed by surveys such as Gaia or near-infrared spectroscopic surveys, which have good coverage of the Galactic midplane region, is highly complex because of the abundant small-scale structure in the three-dimensional interstellar dust extinction. We introduce a novel framework for analyzing the importance of small-scale structure in the extinction. This formalism demonstrates that the spatially complex effect of extinction on the selection function of a pencil-beam or contiguous sky survey is equivalent to amore » low-pass filtering of the extinction-affected selection function with the smooth density field. We find that the angular resolution of current 3D extinction maps is sufficient for analyzing Gaia sub-samples of millions of stars. However, the current distance resolution is inadequate and needs to be improved by an order of magnitude, especially in the inner Galaxy. We also present a practical and efficient method for properly taking the effect of extinction into account in analyses of Galactic structure through an effective selection function. We illustrate its use with the selection function of red-clump stars in APOGEE using and comparing a variety of current 3D extinction maps.« less

The VLT-FLAMES Tarantula Survey. IX. The interstellar medium seen through diffuse interstellar bands and neutral sodium&

NASA Astrophysics Data System (ADS)

van Loon, J. Th.; Bailey, M.; Tatton, B. L.; Maíz Apellániz, J.; Crowther, P. A.; de Koter, A.; Evans, C. J.; Hénault-Brunet, V.; Howarth, I. D.; Richter, P.; Sana, H.; Simón-Díaz, S.; Taylor, W.; Walborn, N. R.

2013-02-01

Context. The Tarantula Nebula (a.k.a. 30 Dor) is a spectacular star-forming region in the Large Magellanic Cloud (LMC), seen through gas in the Galactic disc and halo. Diffuse interstellar bands (DIBs) offer a unique probe of the diffuse, cool-warm gas in these regions. Aims: The aim is to use DIBs as diagnostics of the local interstellar conditions, whilst at the same time deriving properties of the yet-unknown carriers of these enigmatic spectral features. Methods: Spectra of over 800 early-type stars from the Very Large Telescope Flames Tarantula Survey (VFTS) were analysed. Maps were created, separately, for the Galactic and LMC absorption in the DIBs at 4428 and 6614 Å and - in a smaller region near the central cluster R 136 - neutral sodium (the Na i D doublet); we also measured the DIBs at 5780 and 5797 Å. Results: The maps show strong 4428 and 6614 Å DIBs in the quiescent cloud complex to the south of 30 Dor but weak absorption in the harsher environments to the north (bubbles) and near the OB associations. The Na maps show at least five kinematic components in the LMC and a shell-like structure surrounding R 136, and small-scale structure in the Milky Way. The strengths of the 4428, 5780, 5797 and 6614 Å DIBs are correlated, also with Na absorption and visual extinction. The strong 4428 Å DIB is present already at low Na column density but the 6614, 5780 and 5797 Å DIBs start to be detectable at subsequently larger Na column densities. Conclusions: The carriers of the 4428, 6614, 5780 and 5797 Å DIBs are increasingly prone to removal from irradiated gas. The relative strength of the 5780 and 5797 Å DIBs clearly confirm the Tarantula Nebula as well as Galactic high-latitude gas to represent a harsh radiation environment. The resilience of the 4428 Å DIB suggests its carrier is large, compact and neutral. Structure is detected in the distribution of cool-warm gas on scales between one and >100 pc in the LMC and as little as 0.01 pc in the Sun's vicinity. Stellar winds from the central cluster R 136 have created an expanding shell; some infalling gas is also detected, reminiscent of a galactic "fountain". Full Tables A.2-A.4 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/550/A108

Interstellar gas phase abundance of carbon, oxygen, nitrogen, copper, gallium, germanium, and krypton toward Zeta Ophiuchi

NASA Technical Reports Server (NTRS)

Cardelli, Jason A.; Savage, Blair D.; Ebbets, Dennis C.

1991-01-01

An analysis of weak (less than 10 mA) UV interstellar absorption line data obtained for the line of sight to the O9.5 IV star Zeta Oph is presented. Measurements of weak semiforbidden lines of N I, O I, Cu II, and a new UV detection of Na I are reported along with a small upper limit for C II. Interstellar detections of Ga II, Ge II, and Kr I are also presented. Ga, Ge, and Kr represent the heaviest elements detected in the ISM. A comparison of the derived column densities to cosmic abundances shows Ga to be depleted by about -1.2 dex while Ge is overabundant by +0.2 dex. Assuming Kr to be undepleted, a logarithmic cosmic abundance of Kr/H = 2.95 is obtained on the scale where H = 12.00.

The structure and statistics of interstellar turbulence

NASA Astrophysics Data System (ADS)

Kritsuk, A. G.; Ustyugov, S. D.; Norman, M. L.

2017-06-01

We explore the structure and statistics of multiphase, magnetized ISM turbulence in the local Milky Way by means of driven periodic box numerical MHD simulations. Using the higher order-accurate piecewise-parabolic method on a local stencil (PPML), we carry out a small parameter survey varying the mean magnetic field strength and density while fixing the rms velocity to observed values. We quantify numerous characteristics of the transient and steady-state turbulence, including its thermodynamics and phase structure, kinetic and magnetic energy power spectra, structure functions, and distribution functions of density, column density, pressure, and magnetic field strength. The simulations reproduce many observables of the local ISM, including molecular clouds, such as the ratio of turbulent to mean magnetic field at 100 pc scale, the mass and volume fractions of thermally stable Hi, the lognormal distribution of column densities, the mass-weighted distribution of thermal pressure, and the linewidth-size relationship for molecular clouds. Our models predict the shape of magnetic field probability density functions (PDFs), which are strongly non-Gaussian, and the relative alignment of magnetic field and density structures. Finally, our models show how the observed low rates of star formation per free-fall time are controlled by the multiphase thermodynamics and large-scale turbulence.

Laboratory rotational spectroscopy of cyano substituted polycyclic aromatic hydrocarbons

NASA Astrophysics Data System (ADS)

McNaughton, Don; Jahn, Michaela K.; Travers, Michael J.; Wachsmuth, Dennis; Godfrey, Peter D.; Grabow, Jens-Uwe

2018-06-01

The rotational spectra of the four cyano substituted polycyclic aromatic hydrocarbon (PAH) molecules 1-cyanonaphthalene, 2-cyanonaphthalene, 9-cyanoanthracene, and 9-cyanophenanthrene have been recorded in molecular expansions using a Stark-modulated millimetre-wave spectrometer and a Fourier transform microwave spectrometer in the centimetre-wave region. The spectra have been assigned and fitted to provide molecular constants and quadrupole hyperfine constants of sufficient accuracy to enable complete hyperfine structure line predictions for interstellar searches. The data may provide a route into detection of small PAHs in the interstellar medium.

Ambipolar diffusion drifts and dynamos in turbulent gases

NASA Technical Reports Server (NTRS)

Zweibel, Ellen G.

1988-01-01

Ambipolar drift in turbulent fluids are considered. Using mean-field electrodynamics, a two-scale theory originally used to study hydromagnetic dynamos, it is shown that magnetic fields can be advected by small-scale magnetosonic (compressional) turbulence or generated by Alfvenic (helical) turbulence. A simple dynamo theory is made and is compared with standard theories in which dissipation is caused by turbulent diffusion. The redistribution of magnetic flux in interstellar clouds is also discussed.

STRUCTURE IN THE ROTATION MEASURE SKY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stil, J. M.; Taylor, A. R.; Sunstrum, C.

2011-01-01

An analysis of structure in rotation measure (RM) across the sky based on the RM catalog of Taylor et al. is presented. Several resolved RM structures are identified with structure in the local interstellar medium, including radio loops I, II, and III, the Gum nebula, and the Orion-Eridanus superbubble. Structure functions (SFs) of RM are presented for selected areas, and maps of SF amplitude and slope across the sky are compared with H{alpha} intensity and diffuse polarized intensity. RM variance on an angular scale of 1{sup 0} is correlated with length of the line of sight through the Galaxy, withmore » a contribution from local structures. The slope of the SFs is less concentrated to the Galactic plane and less correlated with length of the line of sight through the Galaxy, suggesting a more local origin for RM structure on angular scales {approx}10{sup 0}. The RM variance is a factor of {approx}2 higher toward the South Galactic Pole than toward the North Galactic Pole, reflecting a more wide-spread asymmetry between the northern and southern Galactic hemispheres. Depolarization of diffuse Galactic synchrotron emission at latitudes <30{sup 0} can be explained largely by Faraday dispersion related to small-scale variance in RM, but the errors allow a significant contribution from differential Faraday rotation along the line of sight.« less

Learning about the very local interstellar medium from the Voyagers

NASA Astrophysics Data System (ADS)

Florinski, V. A.; Guo, X.; Burlaga, L. F.

2017-12-01

The outer heliosheath is the region in front of the heliopause affected by the interaction between the solar wind and the flow of interstellar gas. Voyager 1 has been exploring this region for over five years uncovering a number of remarkable phenomena not present elsewhere in space directly accessible by spacecraft. The very local interstellar medium (VLISM) is characterized by remarkably low levels of magnetic fluctuation intensities presenting a nearly scatter free environment for energetic particle propagation. The fluctuations have power law spectra and probably belong to the inertial range of a turbulent cascade fed by a variety of sources, including large and kinetic scale instabilities and the inner heliosheath structures transmitted across the heliopause. While the fluxes of galactic cosmic rays are, on average, very steady in the VLISM, in agreement with theoretical expectations, they also show episodic depletions at the 90 degree pitch angle. These anisotropy events may be associated with the passage of weak compressive magnetic structures resembling shock waves, but with ramp widths orders of magnitude broader than the relevant kinetic plasma scales. The key to understanding the VLISM lies in recognizing the interconnections between the magnetic field data, which has a "local" character, the highly mobile cosmic rays that sample vast regions of space along magnetic field lines, and neutral atom populations measured by IBEX that can reveal kinetic scale physics of energetic ions produced in charge exchange events.

iWander: Dynamics of interstellar wanderers

NASA Astrophysics Data System (ADS)

Zuluaga, Jorge I.; Sanchez-Hernandez, Oscar; Sucerquia, Mario; Ferrin, Ignacio

2018-01-01

iWander assesses the origin of interstellar small bodies such as asteroids and comets. It includes a series of databases and tools that can be used in general for studying the dynamics of an interstellar vagabond object (small‑body, interstellar spaceship and even stars).

Fractal Structures and Scaling Laws in the Universe:. Statistical Mechanics of the Self-Gravitating Gas

NASA Astrophysics Data System (ADS)

de Vega, H. J.; Sánchez, N.; Combes, F.

2000-09-01

Fractal structures are observed in the universe in two very different ways. Firstly, in the gas forming the cold interstellar medium in scales from 10-4pc till l00pc. Secondly, the galaxy distribution has been observed to be fractal in scales up to hundreds of Mpc. We give here a short review of the statistical mechanical (and field theoretical) approach developed by us for the cold interstellar medium (ISM) and large structure of the universe. We consider a non-relativistic self-gravitating gas in thermal equilibrium at temperature T inside a volume V. The statistical mechanics of such system has special features and, as is known, the thermodynamical limit does not exist in its customary form. Moreover, the treatments through microcanonical, canonical and grand canonical ensembles yield different results. We present here for the first time the equation of state for the self-gravitating gas in the canonical ensemble. We find that it has the form p = [NT/V]f(η), where p is the pressure, N is the number of particles and η ≡ (Gm2 N)/(V1/3 T) The N → ∞ and V → ∞ limit exists keeping η fixed. We compute the function f(η) using Monte Carlo simulations and for small η, analytically. We compute the thermodynamic quantities of the system as free energy, entropy, chemical potential, specific heat, compressibility and speed of sound. We reproduce the well-known gravitational phase transition associated to the Jeans' instability. Namely, a gaseous phase for η < ηc and a condensed phase for η > ηc. Moreover, we derive the precise behaviour of the physical quantities near the transition. In particular, the pressure vanishes as p (ηc - η)B with B 0.2 and ηc 1.6 and the energy fluctuations diverge as (ηc - η)B-1. The speed of sound decreases monotonically with η and approaches the value √ {T/6} at the transition.

The hydrogen coverage of interstellar PAHs

NASA Technical Reports Server (NTRS)

Barker, J. R.; Cohen, M.; Tielens, Alexander G. G. M.; Allamandola, Louis J.; Barker, J. R.; Barker, J. R.

1986-01-01

The rate at which the CH bond in interstellar Polycyclic Aromatic Hydrocarbons (PAHs) rupture due to the absorption of a UV photon has been calculated. The results show that small PAHs (less than or equal to 25 carbon atoms) are expected to be partially dehydrogenated in regions with intense UV fields, while large PAHs (greater than or equal to 25 atoms) are expected to be completely hydrogenated in those regions. Because estimate of the carbon content of interstellar PAHs lie in the range of 20 to 25 carbon atoms, dehydrogenation is probably not very important. Because of the absence of other emission features besides the 11.3 micrometer feature in ground-based 8 to 13 micrometer spectra, it has been suggested that interstellar PAHs are partially dehydrogenated. However, IRAS 8 to 22 micrometer spectra of most sources that show strong 7.7 and 11.2 micrometer emission features also show a plateau of emission extending from about 11.3 to 14 micrometer. Like the 11.3 micrometer feature, this new feature is attributed to the CH out of plane bending mode in PAHs. This new feature shows that interstellar PAHs are not as dehydrogenated as estimated from ground-based 8 to 13 micrometer spectra. It also constrains the molecular structure of interstellar PAHs. In particular, it seems that very condensed PAHs, such as coronene and circumcoronene, dominate the interstellar PAH mixture as expected from stability arguments.

A PORTRAIT OF COLD GAS IN GALAXIES AT 60 pc RESOLUTION AND A SIMPLE METHOD TO TEST HYPOTHESES THAT LINK SMALL-SCALE ISM STRUCTURE TO GALAXY-SCALE PROCESSES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Leroy, Adam K.; Hughes, Annie; Schruba, Andreas

2016-11-01

The cloud-scale density, velocity dispersion, and gravitational boundedness of the interstellar medium (ISM) vary within and among galaxies. In turbulent models, these properties play key roles in the ability of gas to form stars. New high-fidelity, high-resolution surveys offer the prospect to measure these quantities across galaxies. We present a simple approach to make such measurements and to test hypotheses that link small-scale gas structure to star formation and galactic environment. Our calculations capture the key physics of the Larson scaling relations, and we show good correspondence between our approach and a traditional “cloud properties” treatment. However, we argue thatmore » our method is preferable in many cases because of its simple, reproducible characterization of all emission. Using, low- J {sup 12}CO data from recent surveys, we characterize the molecular ISM at 60 pc resolution in the Antennae, the Large Magellanic Cloud (LMC), M31, M33, M51, and M74. We report the distributions of surface density, velocity dispersion, and gravitational boundedness at 60 pc scales and show galaxy-to-galaxy and intragalaxy variations in each. The distribution of flux as a function of surface density appears roughly lognormal with a 1 σ width of ∼0.3 dex, though the center of this distribution varies from galaxy to galaxy. The 60 pc resolution line width and molecular gas surface density correlate well, which is a fundamental behavior expected for virialized or free-falling gas. Varying the measurement scale for the LMC and M31, we show that the molecular ISM has higher surface densities, lower line widths, and more self-gravity at smaller scales.« less

Heliospheric influence on the anisotropy of TeV cosmic rays

DOE PAGES

Zhang, Ming; Zuo, Pingbing; Pogorelov, Nikolai

2014-06-26

This article provides a theory of using Liouville's theorem to map the anisotropy of TeV cosmic rays seen at Earth using the particle distribution function in the local interstellar medium (LISM). The ultimate source of cosmic ray anisotropy is the energy, pitch angle, and spatial dependence of the cosmic ray distribution function in the LISM. Because young nearby cosmic ray sources can make a special contribution to the cosmic ray anisotropy, the anisotropy depends on the source age, distance and magnetic connection, and particle diffusion of these cosmic rays, all of which make the anisotropy sensitive to the particle energy.more » When mapped through the magnetic and electric field of a magnetohydrodynamic model heliosphere, the large-scale dipolar and bidirectional interstellar anisotropy patterns become distorted if they are seen from Earth, resulting in many small structures in the observations. Best fits to cosmic ray anisotropy measurements have allowed us to estimate the particle density gradient and pitch angle anisotropies in the LISM. It is found that the heliotail, hydrogen deflection plane, and the plane perpendicular to the LISM magnetic field play a special role in distorting cosmic ray anisotropy. These features can lead to an accurate determination of the LISM magnetic field direction and polarity. The effects of solar cycle variation, the Sun's coronal magnetic field, and turbulence in the LISM and heliospheric magnetic fields are minor but clearly visible at a level roughly equal to a fraction of the overall anisotropy amplitude. Lastly, the heliospheric influence becomes stronger at lower energies. Below 1 TeV, the anisotropy is dominated by small-scale patterns produced by disturbances in the heliosphere.« less

Heliospheric influence on the anisotropy of TeV cosmic rays

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Ming; Zuo, Pingbing; Pogorelov, Nikolai, E-mail: mzhang@fit.edu

2014-07-20

This paper provides a theory of using Liouville's theorem to map the anisotropy of TeV cosmic rays seen at Earth using the particle distribution function in the local interstellar medium (LISM). The ultimate source of cosmic ray anisotropy is the energy, pitch angle, and spatial dependence of the cosmic ray distribution function in the LISM. Because young nearby cosmic ray sources can make a special contribution to the cosmic ray anisotropy, the anisotropy depends on the source age, distance and magnetic connection, and particle diffusion of these cosmic rays, all of which make the anisotropy sensitive to the particle energy.more » When mapped through the magnetic and electric field of a magnetohydrodynamic model heliosphere, the large-scale dipolar and bidirectional interstellar anisotropy patterns become distorted if they are seen from Earth, resulting in many small structures in the observations. Best fits to cosmic ray anisotropy measurements have allowed us to estimate the particle density gradient and pitch angle anisotropies in the LISM. It is found that the heliotail, hydrogen deflection plane, and the plane perpendicular to the LISM magnetic field play a special role in distorting cosmic ray anisotropy. These features can lead to an accurate determination of the LISM magnetic field direction and polarity. The effects of solar cycle variation, the Sun's coronal magnetic field, and turbulence in the LISM and heliospheric magnetic fields are minor but clearly visible at a level roughly equal to a fraction of the overall anisotropy amplitude. The heliospheric influence becomes stronger at lower energies. Below 1 TeV, the anisotropy is dominated by small-scale patterns produced by disturbances in the heliosphere.« less

Photodissociation Regions in the Interstellar Medium of Galaxies

NASA Technical Reports Server (NTRS)

Hollenbach, David J.; Tielens, A. G. G. M.; DeVincenzi, Donald L. (Technical Monitor)

1999-01-01

The interstellar medium of galaxies is the reservoir out of which stars are born and into which stars inject newly created elements as they age. The physical properties of the interstellar medium are governed in part by the radiation emitted by these stars. Far-ultraviolet (6 eV less than h(nu) less than 13.6 eV) photons from massive stars dominate the heating and influence the chemistry of the neutral atomic gas and much of the molecular gas in galaxies. Predominantly neutral regions of the interstellar medium in which the heating and chemistry are regulated by far ultraviolet photons are termed Photo-Dissociation Regions (PDRs). These regions are the origin of most of the non-stellar infrared (IR) and the millimeter and submillimeter CO emission from galaxies. The importance of PDRs has become increasingly apparent with advances in IR and submillimeter astronomy. The IR emission from PDRs includes fine structure lines of C, C+, and O; rovibrational lines of H2, rotational lines of CO; broad middle features of polycyclic aromatic hydrocarbons; and a luminous underlying IR continuum from interstellar dust. The transition of H to H2 and C+ to CO occurs within PDRs. Comparison of observations with theoretical models of PDRs enables one to determine the density and temperature structure, the elemental abundances, the level of ionization, and the radiation field. PDR models have been applied to interstellar clouds near massive stars, planetary nebulae, red giant outflows, photoevaporating planetary disks around newly formed stars, diffuse clouds, the neutral intercloud medium, and molecular clouds in the interstellar radiation field-in summary, much of the interstellar medium in galaxies. Theoretical PDR models explain the observed correlations of the [CII] 158 microns with the COJ = 1-0 emission, the COJ = 1-0 luminosity with the interstellar molecular mass, and the [CII] 158 microns plus [OI] 63 microns luminosity with the IR continuum luminosity. On a more global scale, MR models predict the existence of two stable neutral phases of the interstellar medium, elucidate the formation and destruction of star-forming molecular clouds, and suggest radiation-induced feedback mechanisms that may regulate star formation rates and the column density of gas through giant molecular clouds.

High energy from space

NASA Technical Reports Server (NTRS)

Margon, Bruce; Canizares, Claude; Catura, Richard C.; Clark, George W.; Fichtel, Carl E.; Friedman, Herbert; Giacconi, Riccardo; Grindlay, Jonathan E.; Helfand, David J.; Holt, Stephen S.

1991-01-01

The following subject areas are covered: (1) important scientific problems for high energy astrophysics (stellar activity, the interstellar medium in galaxies, supernovae and endpoints of stellar evolution, nucleosynthesis, relativistic plasmas and matter under extreme conditions, nature of gamma-bursts, identification of black holes, active nuclei, accretion physics, large-scale structures, intracluster medium, nature of dark matter, and the X- and gamma-ray background); (2) the existing experimental programs (Advanced X-Ray Astrophysics Facility (AXAF), Gamma Ray Observatory (GRO), X-Ray Timing Explorer (XTE), High Energy Transient Experiment (HETE), U.S. participation in foreign missions, and attached Shuttle and Space Station Freedom payloads); (3) major missions for the 1990's; (4) a new program of moderate missions; (5) new opportunities for small missions; (6) technology development issues; and (7) policy issues.

ON THE GEOMETRY OF THE IBEX RIBBON

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sylla, Adama; Fichtner, Horst

2015-10-01

The Energetic Neutral Atom (ENA) full-sky maps obtained with the Interstellar Boundary Explorer (IBEX) show an unexpected bright narrow band of increased intensity. This so-called ENA ribbon results from charge exchange of interstellar neutral atoms with protons in the outer heliosphere or beyond. Among other hypotheses it has been argued that this ribbon may be related to a neutral density enhancement, or H-wave, in the local interstellar medium. Here we quantitatively demonstrate, on the basis of an analytical model of the principal large-scale heliospheric structure, that this scenario for the ribbon formation leads to results that are fully consistent withmore » the observed location of the ribbon in the full-sky maps at all energies detected with high-energy sensor IBEX-Hi.« less

Spatial Transport of Magnetic Flux Surfaces in Strongly Anisotropic Turbulence

NASA Astrophysics Data System (ADS)

Matthaeus, W. H.; Servidio, S.; Wan, M.; Ruffolo, D. J.; Rappazzo, A. F.; Oughton, S.

2013-12-01

Magnetic flux surfaces afford familiar descriptions of spatial structure, dynamics, and connectivity of magnetic fields, with particular relevance in contexts such as solar coronal flux tubes, magnetic field connectivity in the interplanetary and interstellar medium, as well as in laboratory plasmas and dynamo problems [1-4]. Typical models assume that field-lines are orderly, and flux tubes remain identifiable over macroscopic distances; however, a previous study has shown that flux tubes shred in the presence of fluctuations, typically losing identity after several correlation scales [5]. Here, the structure of magnetic flux surfaces is numerically investigated in a reduced magnetohydrodynamic (RMHD) model of homogeneous turbulence. Short and long-wavelength behavior is studied statistically by propagating magnetic surfaces along the mean field. At small scales magnetic surfaces become complex, experiencing an exponential thinning. At large scales, instead, the magnetic flux undergoes a diffusive behavior. The link between the diffusion of the coarse-grained flux and field-line random walk is established by means of a multiple scale analysis. Both large and small scales limits are controlled by the Kubo number. These results have consequences for understanding and interpreting processes such as magnetic reconnection and field-line diffusion in plasmas [6]. [1] E. N. Parker, Cosmical Magnetic Fields (Oxford Univ. Press, New York, 1979). [2] J. R. Jokipii and E. N. Parker, Phys. Rev. Lett. 21, 44 (1968). [3] R. Bruno et al., Planet. Space Sci. 49, 1201 (2001). [4] M. N. Rosenbluth et al., Nuclear Fusion 6, 297 (1966). [5] W. H. Matthaeus et al., Phys. Rev. Lett. 75, 2136 (1995). [6] S. Servidio et al., submitted (2013).

Peering Through the Muck: Notes on the the Influence of the Galactic Interstellar Medium on Extragalactic Observations

NASA Astrophysics Data System (ADS)

Lockman, Felix J.

This paper considers some effects of foreground Galactic gas on radiation received from extragalactic objects, with an emphasis on the use of the 21cm line to determine the total N(HI). In general, the opacity of the 21cm line makes it impossible to derive an accurate value of N(HI) by simply applying a formula to the observed emission, except in directions where there is very little interstellar matter. The 21cm line can be used to estimate the likelihood that there is significant molecular hydrogen in a particular direction, but carries little or no information on the amount of ionized gas, which can be a major source of foreground effects. Considerable discussion is devoted to the importance of small-scale angular structure in HI, with the conclusion that it will rarely contribute significantly to the total error compared to other factors (such as the effects of ionized gas) for extragalactic sight lines at high Galactic latitude. The direction of the Hubble/Chandra Deep Field North is used as an example of the complexities that might occur even in the absence of opacity or molecular gas.

Exact law for homogeneous compressible Hall magnetohydrodynamics turbulence

NASA Astrophysics Data System (ADS)

Andrés, N.; Galtier, S.; Sahraoui, F.

2018-01-01

We derive an exact law for three-dimensional (3D) homogeneous compressible isothermal Hall magnetohydrodynamic turbulence, without the assumption of isotropy. The Hall current is shown to introduce new flux and source terms that act at the small scales (comparable or smaller than the ion skin depth) to significantly impact the turbulence dynamics. The law provides an accurate means to estimate the energy cascade rate over a broad range of scales covering the magnetohydrodynamic inertial range and the sub-ion dispersive range in 3D numerical simulations and in in situ spacecraft observations of compressible turbulence. This work is particularly relevant to astrophysical flows in which small-scale density fluctuations cannot be ignored such as the solar wind, planetary magnetospheres, and the interstellar medium.

Radio astronomy

NASA Technical Reports Server (NTRS)

Kellermann, Kenneth I.; Heeschen, David; Backer, Donald C.; Cohen, Marshall H.; Davis, Michael; Depater, Imke; Deyoung, David; Dulk, George A.; Fisher, J. R.; Goss, W. Miller

1991-01-01

The following subject areas are covered: (1) scientific opportunities (millimeter and sub-millimeter wavelength astronomy; meter to hectometer astronomy; the Sun, stars, pulsars, interstellar masers, and extrasolar planets; the planets, asteroids, and comets; radio galaxies, quasars, and cosmology; and challenges for radio astronomy in the 1990's); (2) recommendations for new facilities (the millimeter arrays, medium scale instruments, and small-scale projects); (3) continuing activities and maintenance, upgrading of telescopes and instrumentation; (4) long range programs and technology development; and (5) social, political, and organizational considerations.

Starry Messages - Searching for Signatures of Interstellar Archaeology

NASA Astrophysics Data System (ADS)

Carrigan, R. A., Jr.

Searching for signatures of cosmic-scale archaeological artefacts such as Dyson spheres or Kardashev civilizations is an interesting alternative to conventional SETI. Uncovering such an artifact does not require the intentional transmission of a signal on the part of the originating civilization. This type of search is called interstellar archaeology or sometimes cosmic archaeology . The detection of intelligence elsewhere in the Universe with interstellar archaeology or SETI would have broad implications for science. For example, the constraints of the anthropic principle would have to be loosened if a different type of intelligence was discovered elsewhere. A variety of interstellar archaeology signatures are discussed including non-natural planetary atmospheric constituents, stellar doping with isotopes of nuclear wastes, Dyson spheres, as well as signatures of stellar and galactic-scale engineering. The concept of a Fermi bubble due to interstellar migration is introduced in the discussion of galactic signatures. These potential interstellar archaeological signatures are classified using the Kardashev scale. A modified Drake equation is used to evaluate the relative challenges of finding various sources. With few exceptions interstellar archaeological signatures are clouded and beyond current technological capabilities. However SETI for so-called cultural transmissions and planetary atmosphere signatures are within reach.

Starry messages: Searching for signatures of interstellar archaeology

DOE Office of Scientific and Technical Information (OSTI.GOV)

Carrigan, Richard A., Jr.; /Fermilab

2009-12-01

Searching for signatures of cosmic-scale archaeological artifacts such as Dyson spheres or Kardashev civilizations is an interesting alternative to conventional SETI. Uncovering such an artifact does not require the intentional transmission of a signal on the part of the original civilization. This type of search is called interstellar archaeology or sometimes cosmic archaeology. The detection of intelligence elsewhere in the Universe with interstellar archaeology or SETI would have broad implications for science. For example, the constraints of the anthropic principle would have to be loosened if a different type of intelligence was discovered elsewhere. A variety of interstellar archaeology signaturesmore » are discussed including non-natural planetary atmospheric constituents, stellar doping with isotopes of nuclear wastes, Dyson spheres, as well as signatures of stellar and galactic-scale engineering. The concept of a Fermi bubble due to interstellar migration is introduced in the discussion of galactic signatures. These potential interstellar archaeological signatures are classified using the Kardashev scale. A modified Drake equation is used to evaluate the relative challenges of finding various sources. With few exceptions interstellar archaeological signatures are clouded and beyond current technological capabilities. However SETI for so-called cultural transmissions and planetary atmosphere signatures are within reach.« less

A small-scale dynamo in feedback-dominated galaxies - III. Cosmological simulations

NASA Astrophysics Data System (ADS)

Rieder, Michael; Teyssier, Romain

2017-12-01

Magnetic fields are widely observed in the Universe in virtually all astrophysical objects, from individual stars to entire galaxies, even in the intergalactic medium, but their specific genesis has long been debated. Due to the development of more realistic models of galaxy formation, viable scenarios are emerging to explain cosmic magnetism, thanks to both deeper observations and more efficient and accurate computer simulations. We present here a new cosmological high-resolution zoom-in magnetohydrodynamic (MHD) simulation, using the adaptive mesh refinement technique, of a dwarf galaxy with an initially weak and uniform magnetic seed field that is amplified by a small-scale dynamo (SSD) driven by supernova-induced turbulence. As first structures form from the gravitational collapse of small density fluctuations, the frozen-in magnetic field separates from the cosmic expansion and grows through compression. In a second step, star formation sets in and establishes a strong galactic fountain, self-regulated by supernova explosions. Inside the galaxy, the interstellar medium becomes highly turbulent, dominated by strong supersonic shocks, as demonstrated by the spectral analysis of the gas kinetic energy. In this turbulent environment, the magnetic field is quickly amplified via a SSD process and is finally carried out into the circumgalactic medium by a galactic wind. This realistic cosmological simulation explains how initially weak magnetic seed fields can be amplified quickly in early, feedback-dominated galaxies, and predicts, as a consequence of the SSD process, that high-redshift magnetic fields are likely to be dominated by their small-scale components.

Observational aspects of polycyclic aromatic hydrocarbon charging in the Interstellar Medium

NASA Technical Reports Server (NTRS)

Bakes, E. L. O.; Tielens, Alexander G. G. M.

1995-01-01

We have investigated the charging processes which affect small carbonaceous dust grains and polycyclic aromatic hydrocarbons (PAH's). Because of their high abundance, interstellar PAH molecules can dominate the charge balance of the interstellar medium (ISM), which controls the heating and cooling interstellar gas and interstellar chemistry. We present the results of our model, which compare well with observations and suggest further applications to both laboratory measurements and data obtainable from the KAO.

A cloud/particle model of the interstellar medium - Galactic spiral structure

NASA Technical Reports Server (NTRS)

Levinson, F. H.; Roberts, W. W., Jr.

1981-01-01

A cloud/particle model for gas flow in galaxies is developed that incorporates cloud-cloud collisions and supernovae as dominant local processes. Cloud-cloud collisions are the main means of dissipation. To counter this dissipation and maintain local dispersion, supernova explosions in the medium administer radial snowplow pushes to all nearby clouds. The causal link between these processes is that cloud-cloud collisions will form stars and that these stars will rapidly become supernovae. The cloud/particle model is tested and used to investigate the gas dynamics and spiral structures in galaxies where these assumptions may be reasonable. Particular attention is given to whether large-scale galactic shock waves, which are thought to underlie the regular well-delineated spiral structure in some galaxies, form and persist in a cloud-supernova dominated interstellar medium; this question is answered in the affirmative.

IRAS Colors of the Pleiades

NASA Technical Reports Server (NTRS)

Carey, Sean J.; Shipman, R. F.; Clark, F. O.

1996-01-01

We present large scale images of the infrared emission of the region around the Pleiades using the ISSA data product from the IRAS mission. Residual Zodiacal background and a discontinuity in the image due to the scanning strategy of the satellite necessitated special background subtraction methods. The 60/100 color image clearly shows the heating of the ambient interstellar medium by the cluster. The 12/100 and 25/100 images peak on the cluster as expected for exposure of small dust grains to an enhanced UV radiation field; however, the 25/100 color declines to below the average interstellar value at the periphery of the cluster. Potential causes of the color deficit are discussed. A new method of identifying dense molecular material through infrared emission properties is presented. The difference between the 100 micron flux density and the 60 micron flux density scaled by the average interstellar 60/100 color ratio (Delta I(sub 100) is a sensitive diagnostic of material with embedded heating sources (Delta I(sub 100) less than 0) and cold, dense cores (Delta I(sub 100) greater than 0). The dense cores of the Taurus cloud complex as well as Lynds 1457 are clearly identified by this method, while the IR bright but diffuse Pleiades molecular cloud is virtually indistinguishable from the nearby infrared cirrus.

Organic Chemistry in Space

NASA Technical Reports Server (NTRS)

Charnley, Steven

2009-01-01

Astronomical observations, theoretical modeling, laboratory simulation and analysis of extraterrestrial material have enhanced our knowledge of the inventory of organic matter in the interstellar medium (ISM) and on small bodies such as comets and asteroids (Ehrenfreund & Charnley 2000). Comets, asteroids and their fragments, meteorites and interplanetary dust particles (IDPs), contributed significant amounts of extraterrestrial organic matter to the young Earth. This material degraded and reacted in a terrestrial prebiotic chemistry to form organic structures that may have served as building blocks for life on the early Earth. In this talk I will summarize our current understanding of the organic composition and chemistry of interstellar clouds. Molecules of astrobiological relevance include the building blocks of our genetic material: nucleic acids, composed of subunits such as N-heterocycles (purines and pyrimidines), sugars and amino acids. Signatures indicative of inheritance of pristine and modified interstellar material in comets and meteorites will also be discussed.

Infrared Extinction and Stellar Populations in the Milky Way Midplane

NASA Astrophysics Data System (ADS)

Zasowski, Gail; Majewski, S. R.; Benjamin, R. A.; Nidever, D. L.; Skrutskie, M. F.; Indebetouw, R.; Patterson, R. J.; Meade, M. R.; Whitney, B. A.; Babler, B.; Churchwell, E.; Watson, C.

2012-01-01

The primary laboratory for developing and testing models of galaxy formation, structure, and evolution is our own Milky Way, the closest large galaxy and the only one in which we can resolve large numbers of individual stars. The recent availability of extensive stellar surveys, particularly infrared ones, has enabled precise, contiguous measurement of large-scale Galactic properties, a major improvement over inferences based on selected, but scattered, sightlines. However, our ability to fully exploit the Milky Way as a galactic laboratory is severely hampered by the fact that its midplane and central bulge -- where most of the Galactic stellar mass lies -- is heavily obscured by interstellar dust. Therefore, proper consideration of the interstellar extinction is crucial. This thesis describes a new extinction-correction method (the RJCE method) that measures the foreground extinction towards each star and, in many cases, enables recovery of its intrinsic stellar type. We have demonstrated the RJCE Method's validity and used it to produce new, reliable extinction maps of the heavily-reddened Galactic midplane. Taking advantage of the recovered stellar type information, we have generated maps probing the extinction at different heliocentric distances, thus yielding information on the elusive three-dimensional distribution of the interstellar dust. We also performed a study of the interstellar extinction law itself which revealed variations previously undetected in the diffuse ISM and established constraints on models of ISM grain formation and evolution. Furthermore, we undertook a study of large-scale stellar structure in the inner Galaxy -- the bar(s), bulge(s), and inner spiral arms. We used observed and extinction-corrected infrared photometry to map the coherent stellar features in these heavily-obscured parts of the Galaxy, placing constraints on models of the central stellar mass distribution.

A Test of the Interstellar Boundary EXplorer Ribbon Formation in the Outer Heliosheath

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gamayunov, Konstantin V.; Rassoul, Hamid; Heerikhuisen, Jacob, E-mail: kgamayunov@fit.edu

NASA’s Interstellar Boundary EXplorer ( IBEX ) mission is imaging energetic neutral atoms (ENAs) propagating to Earth from the outer heliosphere and local interstellar medium (LISM). A dominant feature in all ENA maps is a ribbon of enhanced fluxes that was not predicted before IBEX . While more than a dozen models of the ribbon formation have been proposed, consensus has gathered around the so-called secondary ENA model. Two classes of secondary ENA models have been proposed; the first class assumes weak scattering of the energetic pickup protons in the LISM, and the second class assumes strong but spatially localizedmore » scattering. Here we present a numerical test of the “weak scattering” version of the secondary ENA model using our gyro-averaged kinetic model for the evolution of the phase-space distribution of protons in the outer heliosheath. As input for our test, we use distributions of the primary ENAs from our MHD-plasma/kinetic-neutral model of the heliosphere-LISM interaction. The magnetic field spectrum for the large-scale interstellar turbulence and an upper limit for the amplitude of small-scale local turbulence (SSLT) generated by protons are taken from observations by Voyager 1 in the LISM. The hybrid simulations of energetic protons are also used to set the bounding wavenumbers for the spectrum of SSLT. Our test supports the “weak scattering” version. This makes an additional solid step on the way to understanding the origin and formation of the IBEX ribbon and thus to improving our understanding of the interaction between the heliosphere and the LISM.« less

Laboratory Measurements of Celestial Solids

NASA Technical Reports Server (NTRS)

Sievers, A. J.; Beckwith, S. V. W.

1997-01-01

Our experimental study has focused on laboratory measurements of the low temperature optical properties of a variety of astronomically significant materials in the infrared and mm-wave region of the spectrum. Our far infrared measurements of silicate grains with an open structure have produced a variety of unusual results: (1) the low temperature mass opacity coefficient of small amorphous 2MgO(central dot)SiO2 and MgO(central dot)2SiO2 grains are many times larger than the values previously used for interstellar grain material; (2) all of the amorphous silicate grains studied possess the characteristic temperature dependent signature associated with two level systems in bulk glass; and (3) a smaller but nonzero two level temperature dependence signature is also observed for crystalline particles, its physical origin is unclear. These laboratory measurements yield surprisingly large and variable values for the mm-wave absorption coefficients of small silicate particles similar to interstellar grains, and suggest that the bulk absorptivity of interstellar dust at these long wavelengths will not be well known without such studies. Furthermore, our studies have been useful to better understand the physics of the two level absorption process in amorphous and crystalline grains to gain confidence in the wide applicability of these results.

Prospective of Photon Propulsion for Interstellar Flight

NASA Astrophysics Data System (ADS)

Bae, Young K.

Mastering photon propulsion is proposed to be the key to overcoming the limit of the current propulsion technology based on conventional rocketry and potentially opening a new space era. A perspective on photon propulsion is presented here to elucidate that interstellar manned roundtrip flight could be achievable in a century within a frame of exiting scientific principles, once the required existing technologies are further developed. It is shown that the developmental pathway towards the interstellar flight demands not only technological breakthroughs, but consistent long-term world-scale economic interest and investment. Such interest and investment will result from positive financial returns from routine interstellar commutes that can transport highly valuable commodities in a profitable manner. The Photonic Railway, a permanent energy-efficient transportation structure based on the Beamed-Laser Propulsion (BLP) by Forward and the Photonic Laser Thruster (PLT) by the author, is proposed to enable such routine interstellar commutes via Spacetrains. A four-phased evolutionary developmental pathway towards the Interstellar Photonic Railway is proposed. Each phase poses evolutionary, yet daunting, technological and financial challenges that need to be overcome within each time frame of 20 _ 30 years, and is projected to generate multitudes of applications that would lead to sustainable reinvestment into its development. If successfully developed, the Photonic Railway would bring about a quantum leap in the human economic and social interests in space from explorations to terraforming, mining, colonization, and permanent habitation in exoplanets.

Spectroscopy of PAHs with carbon side chains

NASA Astrophysics Data System (ADS)

Rouille, G.; Steglich, M.; Carpentier, Y.; Huisken, F.; Henning, T.

2011-05-01

The presence of polycyclic aromatic hydrocarbons (PAHs) in space has been inferred ever since sp ecific infrared emission bands were interpreted as their collective fingerprint. In parallel, it has been admitted that the famous diffuse interstellar bands (DIBs), which are absorption features observed in the visible wavelength range, are bands belonging to the electronic spectra of free-flying interstellar molecules yet to be identified. As neutral PAHs of medium and large sizes exhibit absorption bands in the range where the DIBs are found, these molecules, which also fulfill other criteria, have been proposed as potential carriers. Studies of small PAHs in solutions have shown that adding an ethynyl side chain (--CCH) to their structure causes their electronic transitions to shift toward longer wavelengths. This fact, added to the observations of interstellar polyynyl radicals, motivated our current research project on PAHs carrying polyynyl side chains. In a first stage, we are measuring the electronic spectra of small PAHs and of their ethynyl and butadiynyl (--CCCCH) derivatives at cryogenic temperatures in rare gas matrices. Then, measurements will be carried out in supersonic jets, providing us with spectra obtained under conditions relevant to the study of free-flying interstellar molecules. The results of IR absorption measurements will be included in our set of new data. As a complement to our laboratory study on the substituted PAHs, quantum chemical calculations are carried out to interprete and simulate their IR and vibronic spectra. We use the density functional theory approach and its time-dependent extension for calculating the electronic ground states and the electronically excited states, respectively. Through the analysis of the new data, it will be determined whether PAHs carrying polyynyl side chains can play a role in interstellar phenomena. The latest results of this on-going project will be presented.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xu, Siyao; Zhang, Bing, E-mail: syxu@pku.edu.cn, E-mail: zhang@physics.unlv.edu

Fast radio bursts (FRBs) have been identified as extragalactic sources that can probe turbulence in the intergalactic medium (IGM) and their host galaxies. To account for the observed millisecond pulses caused by scatter broadening, we examine a variety of possible electron density fluctuation models in both the IGM and the host galaxy medium. We find that a short-wave-dominated power-law spectrum of density, which may arise in highly supersonic turbulence with pronounced local dense structures of shock-compressed gas in the host interstellar medium (ISM), can produce the required density enhancements at sufficiently small scales to interpret the scattering timescale of FRBs.more » This implies that an FRB residing in a galaxy with efficient star formation in action tends to have a broadened pulse. The scaling of the scattering time with the dispersion measure (DM) in the host galaxy varies in different turbulence and scattering regimes. The host galaxy can be the major origin of scatter broadening, but contributes to a small fraction of the total DM. We also find that the sheet-like structure of the density in the host ISM associated with folded magnetic fields in a viscosity-dominated regime of magnetohydrodynamic (MHD) turbulence cannot give rise to strong scattering. Furthermore, valuable insights into the IGM turbulence concerning the detailed spatial structure of density and magnetic field can be gained from the observed scattering timescale of FRBs. Our results favor the suppression of micro-plasma instabilities and the validity of the collisional-MHD description of turbulence properties in the collisionless IGM.« less

Analysis of interstellar fragmentation structure based on IRAS images

NASA Technical Reports Server (NTRS)

Scalo, John M.

1989-01-01

The goal of this project was to develop new tools for the analysis of the structure of densely sampled maps of interstellar star-forming regions. A particular emphasis was on the recognition and characterization of nested hierarchical structure and fractal irregularity, and their relation to the level of star formation activity. The panoramic IRAS images provided data with the required range in spatial scale, greater than a factor of 100, and in column density, greater than a factor of 50. In order to construct a densely sampled column density map of a cloud complex which is both self-gravitating and not (yet?) stirred up much by star formation, a column density image of the Taurus region has been constructed from IRAS data. The primary drawback to using the IRAS data for this purpose is that it contains no velocity information, and the possible importance of projection effects must be kept in mind.

Anomalous Radio-Wave Scattering from Interstellar Plasma Structures

DTIC Science & Technology

2001-03-10

broadening time from the usually expected l~4 scaling law. We brieÑy discuss applications that include scattering of pulses from the Crab pulsar by... pulsars : general È scattering 1. INTRODUCTION Images of scattered radio sources and distorted pulses from pulsars provide some of the most used...distribution of di†racting irregu- larities. Third, observations of the Crab pulsar show anomalous scalings of pulse broadening with frequency. These are

DEVELOPMENT OF THE MODEL OF GALACTIC INTERSTELLAR EMISSION FOR STANDARD POINT-SOURCE ANALYSIS OF FERMI LARGE AREA TELESCOPE DATA

DOE Office of Scientific and Technical Information (OSTI.GOV)

Acero, F.; Ballet, J.; Ackermann, M.

2016-04-01

Most of the celestial γ rays detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope originate from the interstellar medium when energetic cosmic rays interact with interstellar nucleons and photons. Conventional point-source and extended-source studies rely on the modeling of this diffuse emission for accurate characterization. Here, we describe the development of the Galactic Interstellar Emission Model (GIEM), which is the standard adopted by the LAT Collaboration and is publicly available. This model is based on a linear combination of maps for interstellar gas column density in Galactocentric annuli and for the inverse-Compton emission producedmore » in the Galaxy. In the GIEM, we also include large-scale structures like Loop I and the Fermi bubbles. The measured gas emissivity spectra confirm that the cosmic-ray proton density decreases with Galactocentric distance beyond 5 kpc from the Galactic Center. The measurements also suggest a softening of the proton spectrum with Galactocentric distance. We observe that the Fermi bubbles have boundaries with a shape similar to a catenary at latitudes below 20° and we observe an enhanced emission toward their base extending in the north and south Galactic directions and located within ∼4° of the Galactic Center.« less

Development of the Model of Galactic Interstellar Emission for Standard Point-Source Analysis of Fermi Large Area Telescope Data

DOE PAGES

Acero, F.

2016-04-22

Most of the celestial γ rays detected by the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope originate from the interstellar medium when energetic cosmic rays interact with interstellar nucleons and photons. Conventional point and extended source studies rely on the modeling of this diffuse emission for accurate characterization. We describe here the development of the Galactic Interstellar Emission Model (GIEM) that is the standard adopted by the LAT Collaboration and is publicly available. The model is based on a linear combination of maps for interstellar gas column density in Galactocentric annuli and for the inverse Compton emissionmore » produced in the Galaxy. We also include in the GIEM large-scale structures like Loop I and the Fermi bubbles. The measured gas emissivity spectra con rm that the cosmic-ray proton density decreases with Galactocentric distance beyond 5 kpc from the Galactic Center. The measurements also suggest a softening of the proton spectrum with Galactocentric distance. We observe that the Fermi bubbles have boundaries with a shape similar to a catenary at latitudes below 20° and we observe an enhanced emission toward their base extending in the North and South Galactic direction and located within ~4° of the Galactic Center.« less

Development of the Model of Galactic Interstellar Emission for Standard Point-Source Analysis of Fermi Large Area Telescope Data

NASA Technical Reports Server (NTRS)

Acero, F.; Ackermann, M.; Ajello, M.; Albert, A.; Baldini, L.; Ballet, J.; Barbiellini, G.; Bastieri, D.; Bellazzini, R.; Brandt, T. J.;

How the morphology of dusts influences packing density in small solar system bodies

NASA Astrophysics Data System (ADS)

Zangmeister, C.; Radney, J. G.; Zachariah, M. R.

2014-12-01

Large planetary seedlings, comets, and nanoscale soot particles are made from rigid, aggregated subunits that are compacted under low compression into larger structures spanning over 10 orders of magnitude in dimensional space. Here, we demonstrate that the packing density (Φf) of compacted rigid aggregates is independent of spatial scale for systems under weak compaction, a regime that includes small solar system bodies. The Φf of rigid aggregated structures across 6 orders of magnitude were measured using nanoscale spherical soot aerosol composed of aggregates with ≈ 17 nm monomeric subunits and aggregates made from uniform monomeric 6 mm spherical subunits at the macroscale. We find Φf = 0.36 ± 0.02 at both the nano- and macroscale. These values are remarkably similar to qf observed for comet nuclei and measured values of other rigid aggregated systems across a wide variety of spatial and formative conditions. We present a packing model that incorporates the aggregate morphology and show that Φf is independent of both monomer and aggregate size. These observations suggest thatqf of rigid aggregates is independent of spatial dimension across varied formative conditions ranging from interstellar space to pharmaceutical manufacturing.

A General Method for Assessing the Origin of Interstellar Small Bodies: The Case of 1I/2017 U1 (‘Oumuamua)

NASA Astrophysics Data System (ADS)

Zuluaga, Jorge I.; Sánchez-Hernández, Oscar; Sucerquia, Mario; Ferrín, Ignacio

2018-06-01

With the advent of more and deeper sky surveys, the discovery of interstellar small objects entering into the solar system has been finally possible. In 2017 October 19, using observations of the Pan-STARRS survey, a fast moving object, now officially named 1I/2017 U1 (‘Oumuamua), was discovered in a heliocentric unbound trajectory, suggesting an interstellar origin. Assessing the provenance of interstellar small objects is key for understanding their distribution, spatial density, and the processes responsible for their ejection from planetary systems. However, their peculiar trajectories place a limit on the number of observations available to determine a precise orbit. As a result, when its position is propagated ∼105–106 years backward in time, small errors in orbital elements become large uncertainties in position in the interstellar space. In this paper we present a general method for assigning probabilities to nearby stars of being the parent system of an observed interstellar object. We describe the method in detail and apply it for assessing the origin of ‘Oumuamua. A preliminary list of potential progenitors and their corresponding probabilities is provided. In the future, when further information about the object and/or the nearby stars be refined, the probabilities computed with our method can be updated. We provide all the data and codes we developed for this purpose in the form of an open source C/C++/Python package, iWander, which is publicly available at http://github.com/seap-udea/iWander.

The FRIGG project: From intermediate galactic scales to self-gravitating cores

NASA Astrophysics Data System (ADS)

Hennebelle, Patrick

2018-03-01

Context. Understanding the detailed structure of the interstellar gas is essential for our knowledge of the star formation process. Aim. The small-scale structure of the interstellar medium (ISM) is a direct consequence of the galactic scales and making the link between the two is essential. Methods: We perform adaptive mesh simulations that aim to bridge the gap between the intermediate galactic scales and the self-gravitating prestellar cores. For this purpose we use stratified supernova regulated ISM magneto-hydrodynamical simulations at the kpc scale to set up the initial conditions. We then zoom, performing a series of concentric uniform refinement and then refining on the Jeans length for the last levels. This allows us to reach a spatial resolution of a few 10-3 pc. The cores are identified using a clump finder and various criteria based on virial analysis. Their most relevant properties are computed and, due to the large number of objects formed in the simulations, reliable statistics are obtained. Results: The cores' properties show encouraging agreements with observations. The mass spectrum presents a clear powerlaw at high masses with an exponent close to ≃-1.3 and a peak at about 1-2 M⊙. The velocity dispersion and the angular momentum distributions are respectively a few times the local sound speed and a few 10-2 pc km s-1. We also find that the distribution of thermally supercritical cores present a range of magnetic mass-to-flux over critical mass-to-flux ratios, typically between ≃0.3 and 3 indicating that they are significantly magnetized. Investigating the time and spatial dependence of these statistical properties, we conclude that they are not significantly affected by the zooming procedure and that they do not present very large fluctuations. The most severe issue appears to be the dependence on the numerical resolution of the core mass function (CMF). While the core definition process may possibly introduce some biases, the peak tends to shift to smaller values when the resolution improves. Conclusions: Our simulations, which use self-consistently generated initial conditions at the kpc scale, produce a large number of prestellar cores from which reliable statistics can be inferred. Preliminary comparisons with observations show encouraging agreements. In particular the inferred CMFs resemble the ones inferred from recent observations. We stress, however, a possible issue with the peak position shifting with numerical resolution.

Large Interstellar Polarisation Survey. II. UV/optical study of cloud-to-cloud variations of dust in the diffuse ISM

NASA Astrophysics Data System (ADS)

Siebenmorgen, R.; Voshchinnikov, N. V.; Bagnulo, S.; Cox, N. L. J.; Cami, J.; Peest, C.

2018-03-01

It is well known that the dust properties of the diffuse interstellar medium exhibit variations towards different sight-lines on a large scale. We have investigated the variability of the dust characteristics on a small scale, and from cloud-to-cloud. We use low-resolution spectro-polarimetric data obtained in the context of the Large Interstellar Polarisation Survey (LIPS) towards 59 sight-lines in the Southern Hemisphere, and we fit these data using a dust model composed of silicate and carbon particles with sizes from the molecular to the sub-micrometre domain. Large (≥6 nm) silicates of prolate shape account for the observed polarisation. For 32 sight-lines we complement our data set with UVES archive high-resolution spectra, which enable us to establish the presence of single-cloud or multiple-clouds towards individual sight-lines. We find that the majority of these 35 sight-lines intersect two or more clouds, while eight of them are dominated by a single absorbing cloud. We confirm several correlations between extinction and parameters of the Serkowski law with dust parameters, but we also find previously undetected correlations between these parameters that are valid only in single-cloud sight-lines. We find that interstellar polarisation from multiple-clouds is smaller than from single-cloud sight-lines, showing that the presence of a second or more clouds depolarises the incoming radiation. We find large variations of the dust characteristics from cloud-to-cloud. However, when we average a sufficiently large number of clouds in single-cloud or multiple-cloud sight-lines, we always retrieve similar mean dust parameters. The typical dust abundances of the single-cloud cases are [C]/[H] = 92 ppm and [Si]/[H] = 20 ppm.

Properties of interstellar wind leading to shape morphology of the dust surrounding HD 61005

NASA Astrophysics Data System (ADS)

Pástor, P.

2017-08-01

Aims: A structure formed by dust particles ejected from the debris ring around HD 61005 is observed in the scattered light. The main aim here is to constrain interstellar wind parameters that lead to shape morphology in the vicinity of HD 61005 using currently available observational data for the debris ring. Methods: Equation of motion of 2 × 105 dust particles ejected from the debris ring under the action of the electromagnetic radiation, stellar wind, and interstellar wind is solved. A two-dimensional (2D) grid is placed in a given direction for accumulation of the light scattered on the dust particles in order to determine the shape morphology. The interaction of the interstellar wind and the stellar wind is considered. Results: Groups of unknown properties of the interstellar wind that create the observed morphology are determined. A relation between number densities of gas components in the interstellar wind and its relative velocity is found. Variations of the shape morphology caused by the interaction with the interstellar clouds of various temperatures are studied. When the interstellar wind velocity is tilted from debris ring axis a simple relation between the properties of the interstellar wind and an angle between the line of sight and the interstellar wind velocity exists. Dust particles that are most significantly influenced by stellar radiation move on the boundary of observed structure. Conclusions: Observed structure at HD 61005 can be explained as a result of dust particles moving under the action of the interstellar wind. Required number densities or velocities of the interstellar wind are much higher than that of the interstellar wind entering the solar system.

Unbound Young Stellar Systems: Star Formation on the Loose

NASA Astrophysics Data System (ADS)

Gouliermis, Dimitrios A.

2018-07-01

Unbound young stellar systems, the loose ensembles of physically related young bright stars, trace the typical regions of recent star formation in galaxies. Their morphologies vary from small few pc-size associations of newly formed stars to enormous few kpc-size complexes composed of stars few 100 Myr old. These stellar conglomerations are located within the disks and along the spiral arms and rings of star-forming disk galaxies, and they are the active star-forming centers of dwarf and starburst galaxies. Being associated with star-forming regions of various sizes, these stellar structures trace the regions where stars form at various length- and timescales, from compact clusters to whole galactic disks. Stellar associations, the prototypical unbound young systems, and their larger counterparts, stellar aggregates, and stellar complexes, have been the focus of several studies for quite a few decades, with special interest on their demographics, classification, and structural morphology. The compiled surveys of these loose young stellar systems demonstrate that the clear distinction of these systems into well-defined classes is not as straightforward as for stellar clusters, due to their low densities, asymmetric shapes and variety in structural parameters. These surveys also illustrate that unbound stellar structures follow a clear hierarchical pattern in the clustering of their stars across various scales. Stellar associations are characterized by significant sub-structure with bound stellar clusters being their most compact parts, while associations themselves are the brighter denser parts of larger stellar aggregates and stellar complexes, which are members of larger super-structures up to the scale of a whole star-forming galaxy. This structural pattern, which is usually characterized as self-similar or fractal, appears to be identical to that of star-forming giant molecular clouds and interstellar gas, driven mainly by turbulence cascade. In this short review, I make a concise compilation of our understanding of unbound young stellar systems across various environments in the local universe, as it is developed during the last 60 years. I present a factual assessment of the clustering behavior of star formation, as revealed from the assembling pattern of stars across loose stellar structures and its relation to the interstellar medium and the environmental conditions. I also provide a consistent account of the processes that possibly play important role in the formation of unbound stellar systems, compiled from both theoretical and observational investigations on the field.

Dust evolution, a global view I. Nanoparticles, nascence, nitrogen and natural selection … joining the dots

NASA Astrophysics Data System (ADS)

Jones, A. P.

2016-12-01

The role and importance of nanoparticles for interstellar chemistry and beyond is explored within the framework of The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS), focusing on their active surface chemistry, the effects of nitrogen doping and the natural selection of interesting nanoparticle sub-structures. Nanoparticle-driven chemistry, and in particular the role of intrinsic epoxide-type structures, could provide a viable route to the observed gas phase OH in tenuous interstellar clouds en route to becoming molecular clouds. The aromatic-rich moieties present in asphaltenes probably provide a viable model for the structures present within aromatic-rich interstellar carbonaceous grains. The observed doping of such nanoparticle structures with nitrogen, if also prevalent in interstellar dust, could perhaps have important and observable consequences for surface chemistry and the formation of precursor pre-biotic species.

Mixing of the Interstellar and Solar Plasmas at the Heliospheric Interface

DOE PAGES

Pogorelov, N. V.; Borovikov, S. N.

2015-10-12

From the ideal MHD perspective, the heliopause is a tangential discontinuity that separates the solar wind plasma from the local interstellar medium plasma. There are physical processes, however, that make the heliopause permeable. They can be subdivided into kinetic and MHD categories. Kinetic processes occur on small length and time scales, and cannot be resolved with MHD equations. On the other hand, MHD instabilities of the heliopause have much larger scales and can be easily observed by spacecraft. The heliopause may also be a subject of magnetic reconnection. In this paper, we discuss mechanisms of plasma mixing at the heliopausemore » in the context of Voyager 1 observations. Numerical results are obtained with a Multi-Scale Fluid-Kinetic Simulation Suite (MS-FLUKSS), which is a package of numerical codes capable of performing adaptive mesh refinement simulations of complex plasma flows in the presence of discontinuities and charge exchange between ions and neutral atoms. The flow of the ionized component is described with the ideal MHD equations, while the transport of atoms is governed either by the Boltzmann equation or multiple Euler gas dynamics equations. The code can also treat nonthermal ions and turbulence produced by them.« less

Full-sky, High-resolution Maps of Interstellar Dust

NASA Astrophysics Data System (ADS)

Meisner, Aaron Michael

We present full-sky, high-resolution maps of interstellar dust based on data from the Wide-field Infrared Survey Explorer (WISE) and Planck missions. We describe our custom processing of the entire WISE 12 micron All-Sky imaging data set, and present the resulting 15 arcsecond resolution, full-sky map of diffuse Galactic dust emission, free of compact sources and other contaminating artifacts. Our derived 12 micron dust map offers angular resolution far superior to that of all other existing full-sky, infrared dust emission maps, revealing a wealth of small-scale filamentary structure. We also apply the Finkbeiner et al. (1999) two-component thermal dust emission model to the Planck HFI maps. We derive full-sky 6.1 arcminute resolution maps of dust optical depth and temperature by fitting this two-component model to Planck 217-857 GHz along with DIRBE/IRAS 100 micron data. In doing so, we obtain the first ever full-sky 100-3000 GHz Planck-based thermal dust emission model, as well as a dust temperature correction with ~10 times enhanced angular resolution relative to DIRBE-based temperature maps. Analyzing the joint Planck/DIRBE dust spectrum, we show that two-component models provide a better fit to the 100-3000 GHz emission than do single-MBB models, though by a lesser margin than found by Finkbeiner et al. (1999) based on FIRAS and DIRBE. We find that, in diffuse sky regions, our two-component 100-217 GHz predictions are on average accurate to within 2.2%, while extrapolating the Planck Collaboration (2013) single-MBB model systematically underpredicts emission by 18.8% at 100 GHz, 12.6% at 143 GHz and 7.9% at 217 GHz. We calibrate our two-component optical depth to reddening, and compare with reddening estimates based on stellar spectra. We find the dominant systematic problems in our temperature/reddening maps to be zodiacal light on large angular scales and the cosmic infrared background anisotropy on small angular scales. Future work will focus on combining our WISE 12 micron dust map and Planck dust model to create a next-generation, full-sky dust extinction map with angular resolution several times better than Schlegel et al. (1998).

Infrared emission spectra of candidate interstellar aromatic molecules

NASA Technical Reports Server (NTRS)

Schlemmer, S.; Balucani, N.; Wagner, D. R.; Steiner, B.; Saykally, R. J.

1996-01-01

Interstellar dust is responsible, through surface reactions, for the creation of molecular hydrogen, the main component of the interstellar clouds in which new stars form. Intermediate between small, gas-phase molecules and dust are the polycyclic aromatic hydrocarbons (PAHs). Such molecules could account for 2-30% of the carbon in the Galaxy, and may provide nucleation sites for the formation of carbonaceous dust. Although PAHs have been proposed as the sources of the unidentified infrared emission bands that are observed in the spectra of a variety of interstellar sources, the emission characteristics of such molecules are still poorly understood. Here we report laboratory emission spectra of several representative PAHs, obtained in conditions approximating those of the interstellar medium, and measured over the entire spectral region spanned by the unidentified infrared bands. We find that neutral PAHs of small and moderate size can at best make only a minor contribution to these emission bands. Cations of these molecules, as well as much larger PAHs and their cations, remain viable candidates for the sources of these bands.

The Ultimate Destination: Choice of Interplanetary Exploration Path can define Future of Interstellar Spaceflight

NASA Astrophysics Data System (ADS)

Silin, D. V.

Manned interstellar spaceflight is facing multiple challenges of great magnitude; among them are extremely large distances and the lack of known habitable planets other than Earth. Many of these challenges are applicable to manned space exploration within the Solar System to the same or lesser degree. If these issues are resolved on an interplanetary scale, better position to pursue interstellar exploration can be reached. However, very little progress (if any) was achieved in manned space exploration since the end of Space Race. There is no lack of proposed missions, but all of them require considerable technological and financial efforts to implement while yielding no tangible benefits that would justify their costs. To overcome this obstacle highest priority in future space exploration plans should be assigned to the creation of added value in outer space. This goal can be reached if reductions in space transportation, construction and maintenance of space-based structures costs are achieved. In order to achieve these requirements several key technologies have to be mastered, such as near-Earth object mining, space- based manufacturing, agriculture and structure assembly. To keep cost and difficulty under control next exploration steps can be limited to nearby destinations such as geostationary orbit, low lunar orbit, Moon surface and Sun-Earth L1 vicinity. Completion of such a program will create a solid foundation for further exploration and colonization of the Solar System, solve common challenges of interplanetary and interstellar spaceflight and create useful results for the majority of human population. Another important result is that perception of suitable destinations for interstellar missions will change significantly. If it becomes possible to create habitable and self-sufficient artificial environments in the nearby interplanetary space, Earth-like habitable planets will be no longer required to expand beyond our Solar System. Large fraction of the stars in the observable Universe will become valid targets for interstellar missions.

Spin-forbidden and spin-allowed cyclopropenone (c-H{sub 2}C{sub 3}O) formation in interstellar medium

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ahmadvand, Seyedsaeid; Zaari, Ryan R.; Varganov, Sergey A., E-mail: svarganov@unr.edu

2014-11-10

Three proposed mechanisms of cyclopropenone (c-H{sub 2}C{sub 3}O) formation from neutral species are studied using high-level electronic structure methods in combination with nonadiabatic transition state and collision theories to deduce the likelihood of each reaction mechanism under interstellar conditions. The spin-forbidden reaction involving the singlet electronic state of cyclopenylidene (c-C{sub 3}H{sub 2}) and the triplet state of atomic oxygen is studied using nonadiabatic transition state theory to predict the rate constant for c-H{sub 2}C{sub 3}O formation. The spin-allowed reactions of c-C{sub 3}H{sub 2} with molecular oxygen and acetylene with carbon monoxide were also investigated. The reaction involving the ground electronicmore » states of acetylene and carbon monoxide has a very large reaction barrier and is unlikely to contribute to c-H{sub 2}C{sub 3}O formation in interstellar medium. The spin-forbidden reaction of c-C{sub 3}H{sub 2} with atomic oxygen, despite the high probability of nonadiabatic transition between the triplet and singlet states, was found to have a very small rate constant due to the presence of a small (3.8 kcal mol{sup –1}) reaction barrier. In contrast, the spin-allowed reaction between c-C{sub 3}H{sub 2} and molecular oxygen is found to be barrierless, and therefore can be an important path to the formation of c-H{sub 2}C{sub 3}O molecule in interstellar environment.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Acero, F.

Most of the celestial γ rays detected by the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope originate from the interstellar medium when energetic cosmic rays interact with interstellar nucleons and photons. Conventional point and extended source studies rely on the modeling of this diffuse emission for accurate characterization. We describe here the development of the Galactic Interstellar Emission Model (GIEM) that is the standard adopted by the LAT Collaboration and is publicly available. The model is based on a linear combination of maps for interstellar gas column density in Galactocentric annuli and for the inverse Compton emissionmore » produced in the Galaxy. We also include in the GIEM large-scale structures like Loop I and the Fermi bubbles. The measured gas emissivity spectra con rm that the cosmic-ray proton density decreases with Galactocentric distance beyond 5 kpc from the Galactic Center. The measurements also suggest a softening of the proton spectrum with Galactocentric distance. We observe that the Fermi bubbles have boundaries with a shape similar to a catenary at latitudes below 20° and we observe an enhanced emission toward their base extending in the North and South Galactic direction and located within ~4° of the Galactic Center.« less

UNDERSTANDING THE STRUCTURE OF THE HOT INTERSTELLAR MEDIUM IN NORMAL EARLY-TYPE GALAXIES.

NASA Astrophysics Data System (ADS)

Traynor, Liam; Kim, Dong-Woo; Chandra Galaxy Atlas

2018-01-01

The hot interstellar medium (ISM) of early-type galaxies (ETG's) provides crucial insight into the understanding of their formation and evolution. Mechanisms such as type Ia supernovae heating, AGN feedback, deepening potential depth through dark matter assembly and ramp-pressure stripping are known to affect the structure of the ISM. By using temperature maps and radial temperature profiles of the hot ISM from ~70 ETG's with archival Chandra data, it is possible to classify the galaxy's ISM into common structural types. This is extended by using 3D fitting of the radial temperature profile in order to provide models that further constrain the structural types. Five structural types are present, negative (temperature decreases with radii), positive (temperature increases with radii), hybrid-dip (temperature decreases at small radii and increases at large radii), hybrid-bump (inverse of hybrid-dip) and quasi-isothermal (temperature is constant at all radii). This work will be continued by 1) determining which mechanisms are present in which galaxies and 2) analysing the model parameters between galaxies within each structural type to determine whether each type can be described by a single set of model parameters, indicating that the same physical processes are responsible for creating that structural type.

Local turbulence simulations for the multiphase ISM

NASA Astrophysics Data System (ADS)

Kissmann, R.; Kleimann, J.; Fichtner, H.; Grauer, R.

2008-12-01

In this paper, we show results of numerical simulations for the turbulence in the interstellar medium (ISM). These results were obtained using a Riemann solver-free numerical scheme for high-Mach number hyperbolic equations. Here, we especially concentrate on the physical properties of the ISM. That is, we do not present turbulence simulations trimmed to be applicable to the ISM. The simulations are rather based on physical estimates for the relevant parameters of the interstellar gas. Applying our code to simulate the turbulent plasma motion within a typical interstellar molecular cloud, we investigate the influence of different equations of state (isothermal and adiabatic) on the statistical properties of the resulting turbulent structures. We find slightly different density power spectra and dispersion maps, while both cases yield qualitatively similar dissipative structures, and exhibit a departure from the classical Kolmogorov case towards a scaling described by the She-Leveque model. Solving the full energy equation with realistic heating/cooling terms appropriate for the diffuse interstellar gas (DIG), we are able to reproduce a realistic two-phase distribution of cold and warm plasma. When extracting maps of polarized intensity from our simulation data, we find encouraging similarity to actual observations. Finally, we compare the actual magnetic field strength of our simulations to its value inferred from the rotation measure. We find these to be systematically different by a factor of about 1.15, thus highlighting the often-underestimated influence of varying line-of-sight particle densities on the magnetic field strength derived from observed rotation measures.

ASSOCIATIONS BETWEEN SMALL-SCALE STRUCTURE IN LOCAL GALACTIC NEUTRAL HYDROGEN AND IN THE COSMIC MICROWAVE BACKGROUND OBSERVED BY PLANCK

DOE Office of Scientific and Technical Information (OSTI.GOV)

Verschuur, Gerrit L., E-mail: gverschu@naic.edu

High-resolution galactic neutral hydrogen (HI) data obtained with the Green Bank Telescope (GBT) over 56 square degrees of sky around l = 132°, b = 25° are compared with small-scale structure in the Cosmic Microwave Background observed by PLANCK, specifically at 143 and 857 GHz, as well as with 100 μm observations from the IRIS survey. The analysis uses data in 13 2° × 2° sub-areas found in the IRSA database at IPAC. The results confirm what has been reported previously; nearby galactic HI features and high-frequency continuum sources believed to be cosmological are in fact clearly associated. While severalmore » attempts strongly suggest that the associations are statistically significant, the key to understanding the phenomenon lies in the fact that in any given area HI is associated with cirrus dust at certain HI velocities and with 143 GHz features at different velocities. At the same time, for the 13 sub-areas studied, there is very little overlap between the dust and 143 GHz features. The data do not imply that the HI itself gives rise to the high-frequency continuum emission. Rather, they appear to indicate undiagnosed brightness enhancements indirectly associated with the HI. If low density interstellar electrons concentrated into clumps, or observed in directions where their integrated line-of-sight column densities are greater than the background in a manner similar to the phenomena that give rise to structure in diffuse HI structure, they will profoundly affect attempts to create a foreground electron mask used for processing PLANCK as well as WMAP data.« less

Primary gamma rays. [resulting from cosmic ray interaction with interstellar matter

NASA Technical Reports Server (NTRS)

Fichtel, C. E.

1974-01-01

Within this galaxy, cosmic rays reveal their presence in interstellar space and probably in source regions by their interactions with interstellar matter which lead to gamma rays with a very characteristic energy spectrum. From the study of the intensity of the high energy gamma radiation as a function of galactic longitude, it is already clear that cosmic rays are almost certainly not uniformly distributed in the galaxy and are not concentrated in the center of the galaxy. The galactic cosmic rays appear to be tied to galactic structural features, presumably by the galactic magnetic fields which are in turn held by the matter in the arm segments and the clouds. On the extragalactic scale, it is now possible to say that cosmic rays are not universal at the density seen near the earth. The diffuse celestial gamma ray spectrum that is observed presents the interesting possibility of cosmological studies and possible evidence for a residual universal cosmic ray density, which is much lower than the present galactic cosmic ray density.

The Heliospheric Termination Shock

NASA Astrophysics Data System (ADS)

Jokipii, J. R.

2013-06-01

The heliospheric termination shock is a vast, spheroidal shock wave marking the transition from the supersonic solar wind to the slower flow in the heliosheath, in response to the pressure of the interstellar medium. It is one of the most-important boundaries in the outer heliosphere. It affects energetic particles strongly and for this reason is a significant factor in the effects of the Sun on Galactic cosmic rays. This paper summarizes the general properties and overall large-scale structure and motions of the termination shock. Observations over the past several years, both in situ and remote, have dramatically revised our understanding of the shock. The consensus now is that the shock is quite blunt, is with the front, blunt side canted at an angle to the flow direction of the local interstellar plasma relative to the Sun, and is dynamical and turbulent. Much of this new understanding has come from remote observations of energetic charged particles interacting with the shock, radio waves and radiation backscattered from interstellar neutral atoms. The observations and the implications are discussed.

The structure of mass-loading shocks. [interaction of solar wind with cometary coma or local interstellar medium using two-fluid model

NASA Technical Reports Server (NTRS)

Zank, G. P.; Khabibrakhmanov, I. KH.; Story, T.

1993-01-01

A new two-fluid model which describes mass loading in the solar wind (e.g., the interaction of the solar wind with a cometary coma or the local interstellar medium) is presented. The self-consistent back-reaction of the mass-loaded ions is included through their effective scattering in low-frequency MHD turbulence and the invocation of a diffusive approximation. Such an approximation has the advantage of introducing self-consistent dissipation coefficients into the governing equations, thereby facilitating the investigation of the internal structure of shocks in mass-loading environments. To illustrate the utility of the new model, we consider the structure of cometary shocks in the hypersonic one-dimensional limit, finding that the incoming solar wind is slowed by both mass loading and the development of a large cometary ion pressure gradient. The shock is broadened and smoothed by the cometary ions with a thickness of the order of the cometary ion diffusion scale.

Steady state and dynamical structure of a cosmic-ray-modified termination shock

NASA Technical Reports Server (NTRS)

Donohue, D. J.; Zank, G. P.

1993-01-01

A hydrodynamic model is developed for the structure of a cosmic-ray-modified termination shock. The model is based on the two-fluid equations of diffuse shock acceleration (Drury and Volk, 1981). Both the steady state structure of the shock and its interaction with outer heliospheric disturbances are considered. Under the assumption that the solar wind is decelerated by diffusing interstellar cosmic rates, it is shown that the natural state of the termination shock is a gradual deceleration and compression, followed by a discontinuous jump to a downstream state which is dominated by the pressure contribution of the cosmic rays. A representative model is calculated for the steady state which incorporates both interstellar cosmic ray mediation and diffusively accelerated anomalous ions through a proposed thermal leakage mechanism. The interaction of large-scale disturbances with the equilibrium termination shock model is shown to result in some unusual downstream structure, including transmitted shocks and cosmic-ray-modified contact discontinuities. The structure observed may be connected to the 2-kHz outer heliospheric radio emission (Cairns et al., 1992a, b). The time-dependent simulations also demonstrate that interaction with solar wind compressible turbulence (e.g., traveling interplanetary shocks, etc.) could induce the termination shock to continually fluctuate between cosmic-ray-dominated and gas-dynamic states. This fluctuation may represent a partial explanation of the galactic cosmic ray modulation effect and illustrates that the Pioneer and Voyager satellites will encounter an evolving shock whose structure and dynamic properties are strongly influence by the mediation of interstellar and anomalous cosmic rays.

Steady state and dynamical structure of a cosmic-ray-modified termination shock

NASA Astrophysics Data System (ADS)

Donohue, D. J.; Zank, G. P.

1993-11-01

A hydrodynamic model is developed for the structure of a cosmic-ray-modified termination shock. The model is based on the two-fluid equations of diffuse shock acceleration (Drury and Volk, 1981). Both the steady state structure of the shock and its interaction with outer heliospheric disturbances are considered. Under the assumption that the solar wind is decelerated by diffusing interstellar cosmic rates, it is shown that the natural state of the termination shock is a gradual deceleration and compression, followed by a discontinuous jump to a downstream state which is dominated by the pressure contribution of the cosmic rays. A representative model is calculated for the steady state which incorporates both interstellar cosmic ray mediation and diffusively accelerated anomalous ions through a proposed thermal leakage mechanism. The interaction of large-scale disturbances with the equilibrium termination shock model is shown to result in some unusual downstream structure, including transmitted shocks and cosmic-ray-modified contact discontinuities. The structure observed may be connected to the 2-kHz outer heliospheric radio emission (Cairns et al., 1992a, b). The time-dependent simulations also demonstrate that interaction with solar wind compressible turbulence (e.g., traveling interplanetary shocks, etc.) could induce the termination shock to continually fluctuate between cosmic-ray-dominated and gas-dynamic states. This fluctuation may represent a partial explanation of the galactic cosmic ray modulation effect and illustrates that the Pioneer and Voyager satellites will encounter an evolving shock whose structure and dynamic properties are strongly influence by the mediation of interstellar and anomalous cosmic rays.

High-energy Gamma Rays from the Milky Way: Three-dimensional Spatial Models for the Cosmic-Ray and Radiation Field Densities in the Interstellar Medium

DOE Office of Scientific and Technical Information (OSTI.GOV)

Porter, T. A.; Moskalenko, I. V.; Jóhannesson, G., E-mail: tporter@stanford.edu

High-energy γ -rays of interstellar origin are produced by the interaction of cosmic-ray (CR) particles with the diffuse gas and radiation fields in the Galaxy. The main features of this emission are well understood and are reproduced by existing CR propagation models employing 2D galactocentric cylindrically symmetrical geometry. However, the high-quality data from instruments like the Fermi Large Area Telescope reveal significant deviations from the model predictions on few to tens of degrees scales, indicating the need to include the details of the Galactic spiral structure and thus requiring 3D spatial modeling. In this paper, the high-energy interstellar emissions frommore » the Galaxy are calculated using the new release of the GALPROP code employing 3D spatial models for the CR source and interstellar radiation field (ISRF) densities. Three models for the spatial distribution of CR sources are used that are differentiated by their relative proportion of input luminosity attributed to the smooth disk or spiral arms. Two ISRF models are developed based on stellar and dust spatial density distributions taken from the literature that reproduce local near- to far-infrared observations. The interstellar emission models that include arms and bulges for the CR source and ISRF densities provide plausible physical interpretations for features found in the residual maps from high-energy γ -ray data analysis. The 3D models for CR and ISRF densities provide a more realistic basis that can be used for the interpretation of the nonthermal interstellar emissions from the Galaxy.« less

A High-Resolution X-Ray and Optical Study of SN1006: Asymmetric Expansion and Small-Scale Structure in a Type Ia Supernova Remnant

NASA Technical Reports Server (NTRS)

Winkler, P. Frank; Williams, Brian J.; Reynolds, Stephen P.; Petre, Robert; Long, Knox S.; Katsuda, Satoru; Hwang, Una

2014-01-01

We introduce a deep (670 ks) X-ray survey of the entire SN 1006 remnant from the Chandra X-Ray Observatory, together with a deep Ha image of SN 1006 from the 4 m Blanco telescope at CTIO. Comparison with Chandra images from 2003 gives the first measurement of the X-ray proper motions around the entire periphery, carried out over a 9 yr baseline. We find that the expansion velocity varies significantly with azimuth. The highest velocity of approx.7400 km/s (almost 2.5 times that in the northwest (NW)) is found along the southeast (SE) periphery, where both the kinematics and the spectra indicate that most of the X-ray emission stems from ejecta that have been decelerated little, if at all. Asymmetries in the distribution of ejecta are seen on a variety of spatial scales. Si-rich ejecta are especially prominent in the SE quadrant, while O and Mg are more uniformly distributed, indicating large-scale asymmetries arising from the explosion itself. Neon emission is strongest in a sharp filament just behind the primary shock along the NWrim, where the pre-shock density is highest. Here the Ne is likely interstellar, while Ne within the shell may include a contribution from ejecta. Within the interior of the projected shell we find a few isolated "bullets" of what appear to be supernova ejecta that are immediately preceded by bowshocks seen in Ha, features that we interpret as ejecta knots that have reached relatively dense regions of the surrounding interstellar medium, but that appear in the interior in projection. Recent three-dimensional hydrodynamic models for Type Ia supernovae display small-scale features that strongly resemble the ones seen in X-rays in SN 1006; an origin in the explosion itself or from subsequent hydrodynamic instabilities both remain viable options. We have expanded the search for precursor X-ray emission ahead of a synchrotron-dominated shock front, as expected from diffusive shock acceleration theory, to numerous regions along both the northeast and southwest rims of the shell. Our data require that a precursor be thinner than about 3, and fainter than about 5% of the post-shock peak. These limits suggest that the magnetic field is amplified by a factor of seven or more in a narrow precursor region, promoting diffusive particle acceleration.

Stellar and interstellar K lines - Gamma Pegasi and iota Herculis.

NASA Technical Reports Server (NTRS)

Hobbs, L. M.

1973-01-01

High-resolution scans show that the relatively strong (about 90 mA) K lines of Ca II in the early B stars gamma-Peg and iota-Her are almost entirely stellar in origin, although the latter case includes a small interstellar contribution. Such stellar lines can be of great importance in augmenting the interstellar absorption, up through the earliest of the B stars.

Thermodynamics and Charging of Interstellar Iron Nanoparticles

NASA Astrophysics Data System (ADS)

Hensley, Brandon S.; Draine, B. T.

2017-01-01

Interstellar iron in the form of metallic iron nanoparticles may constitute a component of the interstellar dust. We compute the stability of iron nanoparticles to sublimation in the interstellar radiation field, finding that iron clusters can persist down to a radius of ≃4.5 Å, and perhaps smaller. We employ laboratory data on small iron clusters to compute the photoelectric yields as a function of grain size and the resulting grain charge distribution in various interstellar environments, finding that iron nanoparticles can acquire negative charges, particularly in regions with high gas temperatures and ionization fractions. If ≳10% of the interstellar iron is in the form of ultrasmall iron clusters, the photoelectric heating rate from dust may be increased by up to tens of percent relative to dust models with only carbonaceous and silicate grains.

The violent interstellar medium in Milky-Way like disk galaxies

NASA Astrophysics Data System (ADS)

Karoline Walch, Stefanie

2015-08-01

Molecular clouds are cold, dense, and turbulent filamentary structures that condense out of the multi-phase interstellar medium. They are also the sites of star formation. The minority of new-born stars is massive, but these stars are particularly important for the fate of their parental molecular clouds as their feedback drives turbulence and regulates star formation.I will present results from the SILCC project (SImulating the Life Cycle of molecular Clouds), in which we study the formation and dispersal of molecular clouds within the multi-phase ISM using high-performance, three-dimensional simulations of representative pieces of disk galaxies. Apart from stellar feedback, self-gravity, an external stellar potential, and magnetic fields, we employ an accurate description of gas heating and cooling as well as a small chemical network including molecule formation and (self-)shielding from the interstellar radiation field. We study the impact of the supernova rate and the positioning of the supernova explosions with respect to the molecular gas in a well defined set of simulations. This allows us to draw conclusions on structure of the multi-phase ISM, the amount of molecular gas formed, and the onset of galactic outflows. Furthermore, we show how important stellar wind feedback is for regulating star formation in these disks.

Turbulence in space plasmas and beyond

NASA Astrophysics Data System (ADS)

Galtier, S.

2018-07-01

Most of the visible matter in the Universe is in the form of highly turbulent plasmas. For a long time the turbulent character of astrophysical fluids has been neglected and not well understood. One reason for this is the extremely complicated physics involved in astrophysical processes ranging from the machinery of stars, solar and stellar winds, accretion disks to interstellar clouds and galaxies. The other reason is that turbulence constitutes in itself a difficult subject where most of the fundamental results belongs to the incompressible hydrodynamics. Nevertheless, significant theoretical progress has been made during the last years to incorporate some ingredients like compressibility or small-scale plasma physics which are fundamental in astrophysics. This paper reviews some of these results with a strong focus on space plasmas (solar wind, solar corona). Turbulence in interstellar clouds (supersonic flows) and cosmology (space-time fluctuations) are also briefly mentioned.

A New Vision of Science and Strategy for an Interstellar Probe Mission

NASA Astrophysics Data System (ADS)

Gruntman, M.; McNutt, R. L.; Krimigis, S. M.; Wimmer-Schweingruber, R. F.; Gold, R. E.

2011-12-01

The recent in-situ and remote observations from the Voyager Interstellar Mission (VIM), the Interstellar Boundary Explorer (IBEX), and Ion and Neutral Camera (INCA) Cassini have revealed the interaction of the heliosphere with the very local interstellar medium (VLISM) to be much more complex than described by our present day concepts. These discoveries call for a major revision of the strategy for the Interstellar Probe, a mission to explore the interstellar medium surrounding the Solar System. Voyager 1 and Voyager 2 continue to reveal unanticipated flow patterns and significant fluxes of energetic particles in the heliosheath (beyond the solar wind termination shock) while pointing to a more remote location for the modulation region and source of the anomalous cosmic rays (ACRs). Remarkably, Voyager 1 has been reporting near-zero plasma flows (10's of km/s) beyond 115 AU for over the past year. One implication of this flow stagnation is that Voyager is already in a "transition layer" that could lead to the interstellar plasma. Consequently an Interstellar Probe Mission may "punch out" into the deflected interstellar plasma flow at a much smaller distance than previous models had predicted. Global imaging observations by IBEX and INCA of energetic neutral atoms (ENAs) originating from the interaction region(s) of the solar wind and the VLISM show unexpected structure and possible time dependence on a variety of scales. In addition to the general "glow" of the sky in ENAs, IBEX revealed a relatively narrow "ribbon" of enhanced atomic hydrogen emission from ~200 eV to ~6 keV. The neutrals from both the glow and ribbon are also characterized by non-thermal distribution functions. In addition, INCA on Cassini sees a "belt" of emission in ENAs, broader than the ribbon and tilted significantly away from it, at even higher energies (10s of keV). This evidence supports the idea that the bulk of the energy density in the heliosheath plasma resides in a non-thermal component that extends to very high energies. We have never sampled such a huge and dynamic plasma regime that is dominated by non-thermal pressure, but it must be representative of the astrospheres of stars similar to our Sun. These new quantitative and qualitative implications for the overall heliospheric structure already call for a new generation of measurements to understand the global nature of our Sun's interaction with the local galactic environment. An interstellar probe with modern instruments and measurement requirements better defined by these recent observations will certainly advance our understanding of the heliospheric interaction and VLISM. New launch vehicles in the evolving fleet, including the Atlas V, Delta IV and Falcon Heavy, offer new capabilities that can enable such a mission with an acceptable development and launch cost.

Delivery of complex organic compounds from evolved stars to the solar system.

PubMed

Kwok, Sun

2011-12-01

Stars in the late stages of evolution are able to synthesize complex organic compounds with aromatic and aliphatic structures over very short time scales. These compounds are ejected into the interstellar medium and distributed throughout the Galaxy. The structures of these compounds are similar to the insoluble organic matter found in meteorites. In this paper, we discuss to what extent stellar organics has enriched the primordial Solar System and possibly the early Earth.

Excess depletion of Al, Ca, Ti from interstellar gas

NASA Technical Reports Server (NTRS)

Clayton, D. D.

1986-01-01

Thermal condensation, cold sticking, and sputtering by interstellar shock are combined with a chemical memory of the condensation sequence to account for depletion of aluminum, calcium, and titanium in interstellar gas. The extra depletion of aluminum and calcium becomes an indicator of the structural history of the refractory parts of interstellar grains.

Evolutionary Models of Cold, Magnetized, Interstellar Clouds

NASA Technical Reports Server (NTRS)

Gammie, Charles F.; Ostriker, Eve; Stone, James M.

2004-01-01

We modeled the long-term and small-scale evolution of molecular clouds using direct 2D and 3D magnetohydrodynamic (MHD) simulations. This work followed up on previous research by our group under auspices of the ATP in which we studied the energetics of turbulent, magnetized clouds and their internal structure on intermediate scales. Our new work focused on both global and smallscale aspects of the evolution of turbulent, magnetized clouds, and in particular studied the response of turbulent proto-cloud material to passage through the Galactic spiral potential, and the dynamical collapse of turbulent, magnetized (supercritical) clouds into fragments to initiate the formation of a stellar cluster. Technical advances under this program include developing an adaptive-mesh MHD code as a successor to ZEUS (ATHENA) in order to follow cloud fragmentation, developing a shearing-sheet MHD code which includes self-gravity and externally-imposed gravity to follow the evolution of clouds in the Galactic potential, and developing radiative transfer models to evaluate the internal ionization of clumpy clouds exposed to external photoionizing UV and CR radiation. Gammie's work at UIUC focused on the radiative transfer aspects of this program.

Recognition and characterization of hierarchical interstellar structure. II - Structure tree statistics

NASA Technical Reports Server (NTRS)

Houlahan, Padraig; Scalo, John

1992-01-01

A new method of image analysis is described, in which images partitioned into 'clouds' are represented by simplified skeleton images, called structure trees, that preserve the spatial relations of the component clouds while disregarding information concerning their sizes and shapes. The method can be used to discriminate between images of projected hierarchical (multiply nested) and random three-dimensional simulated collections of clouds constructed on the basis of observed interstellar properties, and even intermediate systems formed by combining random and hierarchical simulations. For a given structure type, the method can distinguish between different subclasses of models with different parameters and reliably estimate their hierarchical parameters: average number of children per parent, scale reduction factor per level of hierarchy, density contrast, and number of resolved levels. An application to a column density image of the Taurus complex constructed from IRAS data is given. Moderately strong evidence for a hierarchical structural component is found, and parameters of the hierarchy, as well as the average volume filling factor and mass efficiency of fragmentation per level of hierarchy, are estimated. The existence of nested structure contradicts models in which large molecular clouds are supposed to fragment, in a single stage, into roughly stellar-mass cores.

Diffusion of strongly magnetized cosmic ray particles in a turbulent medium

NASA Technical Reports Server (NTRS)

Ptuskin, V. S.

1985-01-01

Cosmic ray (CR) propagation in a turbulent medium is usually considered in the diffusion approximation. Here, the diffusion equation is obtained for strongly magnetized particles in the general form. The influence of a large-scale random magnetic field on CR propagation in interstellar medium is discussed. Cosmic rays are assumed to propagate in a medium with a regular field H and an ensemble of random MHD waves. The energy density of waves on scales smaller than the free path 1 of CR particles is small. The collision integral of the general form which describes interaction between relativistic particles and waves in the quasilinear approximation is used.

THERMODYNAMICS AND CHARGING OF INTERSTELLAR IRON NANOPARTICLES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hensley, Brandon S.; Draine, B. T., E-mail: brandon.s.hensley@jpl.nasa.gov

Interstellar iron in the form of metallic iron nanoparticles may constitute a component of the interstellar dust. We compute the stability of iron nanoparticles to sublimation in the interstellar radiation field, finding that iron clusters can persist down to a radius of ≃4.5 Å, and perhaps smaller. We employ laboratory data on small iron clusters to compute the photoelectric yields as a function of grain size and the resulting grain charge distribution in various interstellar environments, finding that iron nanoparticles can acquire negative charges, particularly in regions with high gas temperatures and ionization fractions. If ≳10% of the interstellar ironmore » is in the form of ultrasmall iron clusters, the photoelectric heating rate from dust may be increased by up to tens of percent relative to dust models with only carbonaceous and silicate grains.« less

Cometary and interstellar dust grains - Analysis by ion microprobe mass spectrometry and other techniques

NASA Technical Reports Server (NTRS)

Zinner, Ernst

1991-01-01

A survey of microanalytical measurements on interplanetary dust particles (IDPs) and interstellar dust grains from primitive meteorites is presented. Ion-microprobe mass spectrometry with its capability to determine isotopic compositions of many elements on a micron spatial scale has played a special role. Examples are measurements of H, N, and O isotopes and refractory trace elements in IDPs; C, N, Mg, and Si isotopes in interstellar SiC grains; and C and N isotopes and H, N, Al, and Si concentrations in interstellar graphite grains.

A high-resolution X-ray image of Puppis A - Inhomogeneities in the interstellar medium

NASA Technical Reports Server (NTRS)

Petre, R.; Kriss, G. A.; Winkler, P. F.; Canizares, C. R.

1982-01-01

Eleven HRI exposures from the Einstein Observatory are assembled into an 0.1-4 keV image of the Puppis A supernova remnant which displays a complex morphology that may reflect the structure of the shocked interstellar medium. In addition to showing a density gradient of a factor greater than four across the approximately 30 pc diameter of the remnant perpendicular to the galactic plane, a shell of X-ray emission is seen surrounding the northern half of Puppis A, coincident with the radio shell, whose edge brightness profile indicates direct hot plasma heating by the blast wave rather than evaporation from clouds. The interior structure of the supernova remnant suggests inhomogeneities whose sizes range over 0.1-5 pc, but with moderate density contrast. Although isolated clouds of 10-30/cu cm density are responsible for the two brightest X-ray features, they represent only a small fraction of the Puppis A mass.

Dust Destruction in the ISM: A Re-Evaluation of Dust Lifetimes

NASA Technical Reports Server (NTRS)

Jones, A. P.; Nuth, J. A., III

2011-01-01

There is a long-standing conundrum in interstellar dust studies relating to the discrepancy between the time-scales for dust formation from evolved stars and the apparently more rapid destruction in supernova-generated shock waves. Aims. We re-examine some of the key issues relating to dust evolution and processing in the interstellar medium. Methods. We use recent and new constraints from observations, experiments, modelling and theory to re-evaluate dust formation in the interstellar medium (ISM). Results. We find that the discrepancy between the dust formation and destruction time-scales may not be as significant as has previously been assumed because of the very large uncertainties involved. Conclusions. The derived silicate dust lifetime could be compatible with its injection time-scale, given the inherent uncertainties in the dust lifetime calculation. The apparent need to re-form significant quantities of silicate dust in the tenuous interstellar medium may therefore not be a strong requirement. Carbonaceous matter, on the other hand, appears to be rapidly recycled in the ISM and, in contrast to silicates, there are viable mechanisms for its re-formation in the ISM.

RESONANT AMPLIFICATION OF TURBULENCE BY THE BLAST WAVES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zankovich, A. M.; Kovalenko, I. G., E-mail: ilya.g.kovalenko@gmail.com

2015-02-10

We discuss the idea of whether spherical blast waves can amplify by a nonlocal resonant hydrodynamic mechanism inhomogeneities formed by turbulence or phase segregation in the interstellar medium. We consider the problem of a blast-wave-turbulence interaction in the Linear Interaction Approximation. Mathematically, this is an eigenvalue problem for finding the structure and amplitude of eigenfunctions describing the response of the shock-wave flow to forced oscillations by external perturbations in the ambient interstellar medium. Linear analysis shows that the blast wave can amplify density and vorticity perturbations for a wide range of length scales with amplification coefficients of up to 20,more » with increasing amplification the larger the length. There also exist resonant harmonics for which the gain becomes formally infinite in the linear approximation. Their orbital wavenumbers are within the range of macro- (l ∼ 1), meso- (l ∼ 20), and microscopic (l > 200) scales. Since the resonance width is narrow (typically, Δl < 1), resonance should select and amplify discrete isolated harmonics. We speculate on a possible explanation of an observed regular filamentary structure of regularly shaped round supernova remnants such as SNR 1572, 1006, or 0509-67.5. Resonant mesoscales found (l ≈ 18) are surprisingly close to the observed scales (l ≈ 15) of ripples in the shell's surface of SNR 0509-67.5.« less

Low-temperature surface formation of NH3 and HNCO: hydrogenation of nitrogen atoms in CO-rich interstellar ice analogues

NASA Astrophysics Data System (ADS)

Fedoseev, G.; Ioppolo, S.; Zhao, D.; Lamberts, T.; Linnartz, H.

2015-01-01

Solid-state astrochemical reaction pathways have the potential to link the formation of small nitrogen-bearing species, like NH3 and HNCO, and prebiotic molecules, specifically amino acids. To date, the chemical origin of such small nitrogen-containing species is still not well understood, despite the fact that ammonia is an abundant constituent of interstellar ices towards young stellar objects and quiescent molecular clouds. This is mainly because of the lack of dedicated laboratory studies. The aim of this work is to experimentally investigate the formation routes of NH3 and HNCO through non-energetic surface reactions in interstellar ice analogues under fully controlled laboratory conditions and at astrochemically relevant temperatures. This study focuses on the formation of NH3 and HNCO in CO-rich (non-polar) interstellar ices that simulate the CO freeze-out stage in dark interstellar cloud regions, well before thermal and energetic processing start to become relevant. We demonstrate and discuss the surface formation of solid HNCO through the interaction of CO molecules with NH radicals - one of the intermediates in the formation of solid NH3 upon sequential hydrogenation of N atoms. The importance of HNCO for astrobiology is discussed.

Large Interstellar Polarisation Survey:The Dust Elongation When Combining Optical-Submm Polarisation

NASA Astrophysics Data System (ADS)

Siebenmorgen, Ralf; Voschinnikov, N.; Bagnulo, S.; Cox, N.; Cami, J.

2017-10-01

The Planck mission has shown that dust properties of the diffuse ISM varies on a large scale and we present variability on a small scales. We present FORS spectro-polarimetry obtained by the Large Interstellar Polarisation Survey along 60 sight-lines. We fit these combined with extinction data by a silicate and carbon dust model with grain sizes ranging from the molecular to the sub-mic. domain. Large silicates of prolate shape account for the observed polarisation. For 37 sight-lines we complement our data set with UVES high-resolution spectra that establish the presence of single or multiple clouds along individual sight-lines. We find correlations between extinction and Serkowski parameters with the dust model and that the presence of multiple clouds depolarises the incoming radiation. However, there is a degeneracy in the dust model between alignment efficiency and the elongation of the grains. This degeneracy can be broken by combining polarization data in the optical-to-submm. This is of wide general interest as it improves the accuracy of deriving dust masses. We show that a flat IR/submm polarisation spectrum with substantial polarisation is predicted from dust models.

Pickup protons and pressure-balanced structures: Voyager 2 observations in merged interaction regions near 35 AU

NASA Astrophysics Data System (ADS)

Burlaga, L. F.; Ness, N. F.; Belcher, J. W.; Szabo, A.; Isenberg, P. A.; Lee, M. A.

1994-11-01

Five pressure-balanced structures, each with a scale of the order of a few hundredths of an astonomical unit (AU), were identified in two merged interaction regions (MIRs) near 35 AU in the Voyager 2 plasma and magnetic field data. They include a tangential discontinuity, simple and complex magnetic holes, slow correlated variations among the plasma and magnetic field parameters, and complex uncorrelated variations among the parameters. The changes in the magnetic pressure in these events are balanced by changes in the pressure of interstellar pickup protons. Thus the pickup protons probably play a major role in the dynamics of the MIRs. The solar wind proton and electron pressures are relatively unimportant in the MIRs at 35 AU and beyond. The region near 35 AU is transition region: the Sun is the source of the magnetic field, but the interstellar medium in source of pickups protons. Relative to the solar wind proton guyroadius, the thicknesses of the discontinuities and simple magnetic holes observed near 35 AU are at least an order of magnitude greater than those observed at 1 AU. However, the thicknesses of the tangential discontinuity and simple magnetic holes observed near 35 AU (in units of the pickup proton Larmor radius) are comparable to those observed at 1 AU (in units of the solar wind proton gyroradius). Thus the gyroradius of interstellar pickup protons controls the thickness of current sheets near 35 AU. We determine the interstellar pickup proton pressure in the PBSs. Using a model for the pickup proton temperature, we estimate that the average interstellar pickup proton pressure, temperature, and density in the MIRs at 35 AU are (0.53 +/- 0.14) x 10-12 erg/cu cm, (5.8 +/- 0.4) x 106 K and (7 +/- 2) x 10-4/cu cm.

Analysis of interstellar cloud structure based on IRAS images

NASA Technical Reports Server (NTRS)

Scalo, John M.

1992-01-01

The goal of this project was to develop new tools for the analysis of the structure of densely sampled maps of interstellar star-forming regions. A particular emphasis was on the recognition and characterization of nested hierarchical structure and fractal irregularity, and their relation to the level of star formation activity. The panoramic IRAS images provided data with the required range in spatial scale, greater than a factor of 100, and in column density, greater than a factor of 50. In order to construct densely sampled column density maps of star-forming clouds, column density images of four nearby cloud complexes were constructed from IRAS data. The regions have various degrees of star formation activity, and most of them have probably not been affected much by the disruptive effects of young massive stars. The largest region, the Scorpius-Ophiuchus cloud complex, covers about 1000 square degrees (it was subdivided into a few smaller regions for analysis). Much of the work during the early part of the project focused on an 80 square degree region in the core of the Taurus complex, a well-studied region of low-mass star formation.

Chemistry in interstellar space. [environment characteristics influencing reaction dynamics

NASA Technical Reports Server (NTRS)

Donn, B.

1973-01-01

The particular characteristics of chemistry in interstellar space are determined by the unique environmental conditions involved. Interstellar matter is present at extremely low densities. Large deviations from thermodynamic equilibrium are, therefore, to be expected. A relatively intense ultraviolet radiation is present in many regions. The temperatures are in the range from 5 to 200 K. Data concerning the inhibiting effect of small activation energies in interstellar clouds are presented in a table. A summary of measured activation energies or barrier heights for exothermic exchange reactions is also provided. Problems of molecule formation are discussed, taking into account gas phase reactions and surface catalyzed processes.

Suppression of Phase Mixing in Drift-Kinetic Plasma Turbulence

NASA Astrophysics Data System (ADS)

Parker, J. T.; Dellar, P. J.; Schekochihin, A. A.; Highcock, E. G.

2017-12-01

The solar wind and interstellar medium are examples of strongly magnetised, weakly collisional, astrophysical plasmas. Their turbulent fluctuations are strongly anisotropic, with small amplitudes, and frequencies much lower than the Larmor frequency. This regime is described by gyrokinetic theory, a reduced five-dimensional kinetic system describing averages over Larmor orbits. A turbulent plasma may transfer free energy, a measure of fluctuation amplitudes, from injection at large scales, typically by an instability, to dissipation at small physical scales like a turbulent fluid. Alternatively, a turbulent plasma may form fine scale structures in velocity space via phase-mixing, the mechanism that leads to Landau damping in linear plasma theory. Macroscopic plasma properties like heat and momentum transport are affected by both mechanisms. While each is understood in isolation, their interaction is not. We study this interaction using a Hankel-Hermite velocity space representation of gyrokinetic theory. The Hankel transform interacts neatly with the Bessel functions that arise from averaging over Larmor orbits, so the perpendicular velocity space is decoupled for linearized problems. The Hermite transform expresses phase mixing as nearest-neighbor coupling between parallel velocity space scales represented by Hermite mode numbers. We use this representation to study transfer mechanisms in drift-kinetic plasma turbulence, the long wavelength limit of gyrokinetic theory. We show that phase space is divided into two regions, with one transfer mechanism dominating in each. Most energy is contained in the region where the fluid-like nonlinear cascade dominates. Moreover, in that region the nonlinear cascade interferes with phase mixing by exciting an "anti phase mixing" transfer of free energy from small to large velocity space scales. This cancels out the usual phase mixing, and renders the overall behavior fluid-like. These results profoundly change our understanding of free energy flow in drift-kinetic turbulence, and, moreover, explain previously observed spectra.

Monte Carlo kinetics simulations of ice-mantle formation on interstellar grains

NASA Astrophysics Data System (ADS)

Garrod, Robin

2015-08-01

The majority of interstellar dust-grain chemical kinetics models use rate equations, or alternative population-based simulation methods, to trace the time-dependent formation of grain-surface molecules and ice mantles. Such methods are efficient, but are incapable of considering explicitly the morphologies of the dust grains, the structure of the ices formed thereon, or the influence of local surface composition on the chemistry.A new Monte Carlo chemical kinetics model, MIMICK, is presented here, whose prototype results were published recently (Garrod 2013, ApJ, 778, 158). The model calculates the strengths and positions of the potential mimima on the surface, on the fly, according to the individual pair-wise (van der Waals) bonds between surface species, allowing the structure of the ice to build up naturally as surface diffusion and chemistry occur. The prototype model considered contributions to a surface particle's potential only from contiguous (or "bonded") neighbors; the full model considers contributions from surface constituents from short to long range. Simulations are conducted on a fully 3-D user-generated dust-grain with amorphous surface characteristics. The chemical network has also been extended from the simple water system previously published, and now includes 33 chemical species and 55 reactions. This allows the major interstellar ice components to be simulated, such as water, methane, ammonia and methanol, as well as a small selection of more complex molecules, including methyl formate (HCOOCH3).The new model results indicate that the porosity of interstellar ices are dependent on multiple variables, including gas density, the dust temperature, and the relative accretion rates of key gas-phase species. The results presented also have implications for the formation of complex organic molecules on dust-grain surfaces at very low temperatures.

Composition, structure and chemistry of interstellar dust

NASA Technical Reports Server (NTRS)

Tielens, Alexander G. G. M.; Allamandola, Louis J.

1986-01-01

The observational constraints on the composition of the interstellar dust are analyzed. The dust in the diffuse interstellar medium consists of a mixture of stardust (amorphous silicates, amorphous carbon, polycyclic aromatic hydrocarbons, and graphite) and interstellar medium dust (organic refractory material). Stardust seems to dominate in the local diffuse interstellar medium. Inside molecular clouds, however, icy grain mantles are also important. The structural differences between crystalline and amorphous materials, which lead to differences in the optical properties, are discussed. The astrophysical consequences are briefly examined. The physical principles of grain surface chemistry are discussed and applied to the formation of molecular hydrogen and icy grain mantles inside dense molecular clouds. Transformation of these icy grain mantles into the organic refractory dust component observed in the diffuse interstellar medium requires ultraviolet sources inside molecular clouds as well as radical diffusion promoted by transient heating of the mantle. The latter process also returns a considerable fraction of the molecules in the grain mantle to the gas phase.

A challenge for probing the statistics of interstellar magnetic fields: beyond the Planck resolution with Herschel

NASA Astrophysics Data System (ADS)

Bracco, Andrea; André, Philippe; Boulanger, Francois

2015-08-01

The recent Planck results in polarization at sub-mm wavelengths allow us to gain insight into the Galactic magnetic field topology, revealing its statistical correlation with matter, from the diffuse interstellar medium (ISM), to molecular clouds (MCs) (Planck intermediate results. XXXII, XXXIII, XXXV). This correlation has a lot to tell us about the dynamics of the turbulent ISM, stressing the importance of considering magnetic fields in the formation of structures, some of which eventually undergo gravitational collapse producing new star-forming cores.Investigating the early phases of star formation has been a fundamental scope of the Herschel Gould Belt survey collaboration (http://gouldbelt-herschel.cea.fr), which, in the last years, has thoroughly characterized, at a resolution of few tens of arcseconds, the statistics of MCs, such as their filamentary structure, kinematics and column density.Although at lower angular resolution, the Planck maps of dust emission at 353GHz, in intensity and polarization, show that all MCs are complex environments, where we observe a non-trivial correlation between the magnetic field and their density structure. This result opens new perspectives on their formation and evolution, which we have started to explore.In this talk, I will present first results of a comparative analysis of the Herschel-Planck data, where we combine the high resolution Herschel maps of some MCs of the Gould Belt with the Planck polarization data, which sample the structure of the field weighted by the density.In particular, I will discuss the large-scale envelopes of the selected MCs, and, given the correlation between magnetic field and matter, I will show how to make use of the high resolution information of the density structure provided by Herschel to investigate the statistics of interstellar magnetic fields in the Planck data.

A bimodal dust grain distribution in the IC 434 H ii region

NASA Astrophysics Data System (ADS)

Ochsendorf, B. B.; Tielens, A. G. G. M.

2015-04-01

Context. Studies of dust evolution and processing in different phases of the interstellar medium (ISM) is essential to understanding the lifecycle of dust in space. Recent results have challenged the capabilities and validity of current dust models, indicating that the properties of interstellar dust evolve as it transits between different phases of the ISM. Aims: We characterize the dust content from the IC 434 H ii region, and present a scenario that results in the large-scale structure of the region seen to date. Methods: We conduct a multi-wavelength study of the dust emission from the ionized gas, and combine this with modeling, from large scales that provide insight into the history of the IC 434/L1630 region, to small scales that allow us to infer quantitative properties of the dust content inside the H ii region. Results: The dust enters the H ii region through momentum transfer with a champagne flow of ionized gas, set up by a chance encounter between the L1630 molecular cloud and the star cluster of σ Ori. We observe two clearly separated dust populations inside the ionized gas, that show different observational properties, as well as contrasting optical properties. Population A is colder (~25 K) than predicted by widely-used dust models, its temperature is insensitive to an increase of the impinging radiation field, it is momentum-coupled to the gas, and efficiently absorbs radiation pressure to form a dust wave at 1.0 pc ahead of σ Ori AB. Population B is characterized by a constant [20/30] flux ratio throughout the H ii region, heats up to ~75 K close to the star, and is less efficient in absorbing radiation pressure, forming a dust wave at 0.1 pc from the star. Conclusions: The dust inside IC 434 is bimodal. The characteristics of population A are remarkable and cannot be explained by current dust models. We argue that large porous grains or fluffy aggregates are potential candidates to explain much of the observational characteristics. Population B are grains that match the classical description of spherical, compact dust. The inferred optical properties are consistent with either very small grains, or large grains in thermal equilibrium with the radiation field. Our results confirm recent work that stress the importance of variations in the dust properties between different regions of the ISM.

On the Plausibility of Pseudosugar Formation in Cometary Ices and Oxygen-rich Tholins

NASA Astrophysics Data System (ADS)

Lavado, Nieves; Ávalos, Martín; Babiano, Reyes; Cintas, Pedro; Light, Mark E.; Jiménez, José Luis; Palacios, Juan C.

2016-03-01

We revisit herein the formation and structure of dihydroxy dioxanes, which can be obtained from prebiotically available precursors and can be regarded as primeval sugar surrogates. Previous studies dealing with the heterogeneous composition of interstellar bodies point to the existence of significant amounts of small polyalcohols along with oxygen-containing oligomers. Even though such derivatives did not give rise to nucleosides and oligonucleotides, nor they were incorporated into subsequent metabolic routes, molecular chimeras based on sugar-like species could be opportunistic scaffolds in pre-evolutionary scenarios. We could figure out that pseudosugars, assembled by hemiacetalic bonds from available precursors in both interstellar and terrestrial scenarios, were presumably more abundant than thought. Moreover, these species share some key features with naturally-occurring sugar rings, such as anomeric preferences, coordinating ability, and the prevalent occurrence of racemic compounds.

Foreword

NASA Astrophysics Data System (ADS)

Boulanger, François; Miville-Deschênes, Marc-Antoine

Scientific awareness of the galactic magnetic field arose in the late 40's with Fermi's work on cosmic rays, the discovery of the polarization of dust reddened starlight by Hiltner and Hall, and its interpretation in terms of magnetically aligned dust grains by Spitzer and Tukey and Davis and Greenstein. It was soon clear that the magnetic field and the cosmic ray are tied to the interstellar gas and form together an active dynamical system. Its importance for star formation and interstellar matter energetics was also quickly recognized but more than 50 years after most questions remain quantitatively open due to the paucity of data on the small scale structure of the field. In the last decade, interferometric surveys have revealed striking patterns in the diffuse polarized emission, imposed by Faraday rotation, that give a way to study the field structure in the intervening ionized medium. The advent of high sensitivity measurements of polarized emission at sub-millimeter wavelengths promise to further open our perspective on the Galactic field. The dust observations will soon extend from high resolution observations from star forming regions with ground based telescopes and the ALMA interferometer to the continuous mapping of the diffuse interstellar medium with Planck. A few months after the discovery of optical interstellar polarization, the Galactic magnetic field was an unexpected highlight of the first symposium on Cosmical Gas Dynamics held in Paris in august 1949. The Polarization 2005 meeting held at the University of Orsay was also a premiere. For the first time, two communities with disconnected backgrounds but common interests in polarization observations were given the opportunity to meet and learn from each other's research. The measurement of the polarization of the Cosmic Microwave Background has become a major Physics challenge of the decade(s) to come. The numerous experiments designed to achieve this goal have started providing data with unprecedented sensitivity on the Galactic polarization from sub-millimeter to centimetric wavelengths. Detailed modeling of the Galactic contribution to the observed polarization is a required key to the success of their endeavor. This book gathers invited lectures presented at the meeting. It gives an in depth account of our present knowledge of the Galactic magnetic field from measurements of its structure and intensity to its role in the physics of interstellar matter and star formation. The Galactic part of the book is complemented by three papers that introduce the Cosmic Microwave Background polarization and describe the experiments being built to observe it, with special attention to the Planck ESA mission. Few years after the Paris Cosmical Gas Dynamics symposium, the discovery of polarized synchrotron emission in the Crab was a new highlight in magnetic field research. Quoting a review by van de Hulst: “From that time on, measuring the polarization and thus mapping the magnetic field became a prime desideratum in galactic radio astronomy”. We hope that observations to come will stimulate similar enthusiasm and bring more and more scientists to the field for whom this book will be a useful and lasting reference. The editors would like to thank the IAS staff that took care of the organization with a special mention for Catherine Cougrand.

The origin of the structure of large-scale magnetic fields in disc galaxies

NASA Astrophysics Data System (ADS)

Nixon, C. J.; Hands, T. O.; King, A. R.; Pringle, J. E.

2018-07-01

The large-scale magnetic fields observed in spiral disc galaxies are often thought to result from dynamo action in the disc plane. However, the increasing importance of Faraday depolarization along any line of sight towards the galactic plane suggests that the strongest polarization signal may come from well above (˜0.3-1 kpc) this plane, from the vicinity of the warm interstellar medium (WIM)/halo interface. We propose (see also Henriksen & Irwin 2016) that the observed spiral fields (polarization patterns) result from the action of vertical shear on an initially poloidal field. We show that this simple model accounts for the main observed properties of large-scale fields. We speculate as to how current models of optical spiral structure may generate the observed arm/interarm spiral polarization patterns.

MULTIFRACTAL STRUCTURES DETECTED BY VOYAGER 1 AT THE HELIOSPHERIC BOUNDARIES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Macek, W. M.; Wawrzaszek, A.; Burlaga, L. F., E-mail: macek@cbk.waw.pl, E-mail: anna.wawrzaszek@cbk.waw.pl, E-mail: lburlagahsp@verizon.net

To better understand the dynamics of turbulent systems, we have proposed a phenomenological model based on a generalized Cantor set with two rescaling and one weight parameters. In this Letter, using recent Voyager 1 magnetic field data, we extend our two-scale multifractal analysis further in the heliosheath beyond the heliospheric termination shock, and even now near the heliopause, when entering the interstellar medium for the first time in human history. We have identified the scaling inertial region for magnetized heliospheric plasma between the termination shock and the heliopause. We also show that the degree of multifractality decreases with the heliocentricmore » distance and is still modulated by the phases of the solar cycle in the entire heliosphere including the heliosheath. Moreover, we observe the change of scaling toward a nonintermittent (nonmultifractal) behavior in the nearby interstellar medium, just beyond the heliopause. We argue that this loss of multifractal behavior could be a signature of the expected crossing of the heliopause by Voyager 2 in the near future. The results obtained demonstrate that our phenomenological multifractal model exhibits some properties of intermittent turbulence in the solar system plasmas, and we hope that it could shed light on universal characteristics of turbulence.« less

The interaction of the halo around the butterfly planetary nebula NGC 650-1 with the interstellar medium

NASA Astrophysics Data System (ADS)

Ramos-Larios, G.; Guerrero, M. A.; Nigoche-Netro, A.; Olguín, L.; Gómez-Muñoz, M. A.; Sabin, L.; Vázquez, R.; Akras, S.; Ramírez Vélez, J. C.; Chávez, M.

2018-03-01

With its bright and wide equatorial waist seen almost edge-on (`the butterfly body') and the faint and broad bipolar extensions (`the butterfly wings'), NGC 650-1 is the archetypical example of bipolar planetary nebula (PN) with butterfly morphology. We present here deep high-resolution broad- and narrow-band optical images that expose the rich and intricate fine structure of this bipolar PN, with small-scale bubble-like features and collimated outflows. A SHAPE spatio-kinematic model indicates that NGC 650-1 has a broad central torus with an inclination angle of 75° with respect to the line of sight, whereas that of the bipolar lobes, which are clearly seen in the position-velocity maps, is 85°. Large field of view deep images show, for first time, an arc-like diffuse envelope in low- and high-excitation emission lines located up to 180 arcsec towards the east-south-east of the central star, well outside the main nebula. This morphological component is confirmed by Spitzer MIPS and WISE infrared imaging, as well as by long-slit low- and high-dispersion optical spectroscopic observations. Hubble Space Telescope images of NGC 650-1 obtained at two different epochs ˜14 yr apart reveal the proper motion of the central star along this direction. We propose that this motion of the star through the interstellar medium compresses the remnant material of a slow asymptotic giant branch wind, producing this bow-shock-like feature.

Numerical Model for Cosmic Rays Species Production and Propagation in the Galaxy

NASA Technical Reports Server (NTRS)

Farahat, Ashraf; Zhang, Ming; Rassoul, Hamid; Connell, J. J.

2005-01-01

In recent years, considerable progress has been made in studying the propagation and origin of cosmic rays, as new and more accurate data have become available. Many models have been developed to study cosmic ray interactions and propagation showed flexibility in resembling various astrophysical conditions and good agreement with observational data. However, some astrophysical problems cannot be addressed using these models, such as the stochastic nature of the cosmic rays source, small-scale structures and inhomogeneities in the interstellar gas that can affect radioactive secondary abundance in cosmic rays. We have developed a new model and a corresponding computer code that can address some of these limitations. The model depends on the expansion of the backward stochastic solution of the general diffusion transport equation (Zhang 1999) starting from an observer position to solve a group of diffusion transport equations each of which represents a particular element or isotope of cosmic ray nuclei. In this paper we are focusing on key abundance ratios such as B/C, sub-Fe/Fe, (10)Be/(9)Be, (26)Al/(27)Al, (36)Cl/(37)Cl and (54)Mn/(55)Mn, which all have well established cross sections, to evaluate our model. The effect of inhomogeneity in the interstellar medium is investigated. The contribution of certain cosmic ray nuclei to the production of other nuclei is addressed. The contribution of various galactic locations to the production of cosmic ray nuclei observed at solar system is also investigated.

The Interstellar Conspiracy

NASA Technical Reports Server (NTRS)

Johnson, Les; Matloff, Gregory L.

2005-01-01

If we were designing a human-carrying starship that could be launched in the not-too-distant future, it would almost certainly not use a warp drive to instantaneously bounce around the universe, as is done in Isaac Asimov's classic Foundation series or in episodes of Star Trek or Star Wars. Sadly, those starships that seem to be within technological reach could not even travel at high relativistic speeds, as does the interstellar ramjet in Poul Anderson's Tau Zero. Warp-speeds seem to be well outside the realm of currently understood physical law; proton-fusing ramjets may never be technologically feasible. Perhaps fortunately in our terrorist-plagued world, the economics of antimatter may never be attractive for large-scale starship propulsion. But interstellar travel will be possible within a few centuries, although it will certainly not be as fast as we might prefer. If humans learn how to hibernate, perhaps we will sleep our way to the stars, as do the crew in A. E. van Vogt's Far Centaurus. However, as discussed in a landmark paper in The Journal of the British Interplanetary Society, the most feasible approach to transporting a small human population to the planets (if any) of Alpha Centauri is the worldship. Such craft have often been featured in science fiction. See for example Arthur C. Clarke's Rendezvous with Rama, and Robert A. Heinlein's Orphans of the Sky. Worldships are essentially mobile versions of the O Neill free-space habitats. Constructed mostly from lunar and/or asteroidal materials, these solar-powered, multi-kilometer-dimension structures could house 10,000 to 100,000 humans in Earth-approximating environments. Artificial gravity would be provided by habitat rotation, and cosmic ray shielding would be provided by passive methods, such as habitat atmosphere and mass shielding, or magnetic fields. A late 21st century space-habitat venture might support itself economically by constructing large solar-powered satellites to beam energy back to Earth.

Structure and evolution of fossil H II regions

NASA Technical Reports Server (NTRS)

Mccray, R.; Schwarz, J.

1971-01-01

The structure and evolution of a fossil H II region created by a burst of ionizing radiation from a supernova is considered. The cooling time scale for the shell is about 10 to the 6th power years. Superposition of million-year-old fossil H II regions may account for the temperature and ionization of the interstellar medium. Fossil H II regions are unstable to growth of thermal condensations. Highly ionized filamentary structures form and dissipate in about 10,000 years. Partially ionized clouds form and dissipate in about 10 to the 6th power years.

Collisional excitation of CH2 rotational/fine-structure levels by helium

NASA Astrophysics Data System (ADS)

Dagdigian, P. J.; Lique, F.

2018-02-01

Accurate determination of the abundance of CH2 in interstellar media relies on both radiative and collisional rate coefficients. We investigate here the rotational/fine-structure excitation of CH2 induced by collisions with He. We employ a recoupling technique to generate fine-structure-resolved cross-sections and rate coefficients from close coupling spin-free scattering calculations. The calculations are based on a recent, high-accuracy CH2-He potential energy surface computed at the coupled clusters level of theory. The collisional cross-section calculations are performed for all fine-structure transitions among the first 22 and 24 energy levels of ortho- and para-CH2, respectively, and for temperatures up to 300 K. As a first application, we simulate the excitation of CH2 in typical molecular clouds. The excitation temperatures of the CH2 lines are found to be small at typical densities of molecular clouds, showing that the non-local thermodynamic equilibrium approach has to be used to analyse interstellar spectra. We also found that the fine-structure lines connected with the 404 - 313 and 505 - 414 rotational transitions show possible maser emissions so that they can be easily seen in emission. These calculations show that CH2 may have to be detected mainly through absorption spectra.

Theory and laboratory astrophysics

NASA Technical Reports Server (NTRS)

Schramm, David N.; Mckee, Christopher F.; Alcock, Charles; Allamandola, Lou; Chevalier, Roger A.; Cline, David B.; Dalgarno, Alexander; Elmegreen, Bruce G.; Fall, S. Michael; Ferland, Gary J.

1991-01-01

Science opportunities in the 1990's are discussed. Topics covered include the large scale structure of the universe, galaxies, stars, star formation and the interstellar medium, high energy astrophysics, and the solar system. Laboratory astrophysics in the 1990's is briefly surveyed, covering such topics as molecular, atomic, optical, nuclear and optical physics. Funding recommendations are given for the National Science Foundation, NASA, and the Department of Energy. Recommendations for laboratory astrophysics research are given.

The LAMAR: A high throughput X-ray astronomy facility for a moderate cost mission

NASA Technical Reports Server (NTRS)

Gorenstein, P.; Schwartz, D.

1981-01-01

The performance of a large area modular array of reflectors (LAMAR) is considered in several hypothetical observations relevant to: (1) cosmology, the X-ray background, and large scale structure of the universe; (2) clusters of galaxies and their evolution; (3) quasars and other active galactic nuclei; (4) compact objects in our galaxy; (5) stellar coronae; and (6) energy input to the interstellar medium.

Matrix isolation as a tool for studying interstellar chemical reactions

NASA Technical Reports Server (NTRS)

Ball, David W.; Ortman, Bryan J.; Hauge, Robert H.; Margrave, John L.

1989-01-01

Since the identification of the OH radical as an interstellar species, over 50 molecular species were identified as interstellar denizens. While identification of new species appears straightforward, an explanation for their mechanisms of formation is not. Most astronomers concede that large bodies like interstellar dust grains are necessary for adsorption of molecules and their energies of reactions, but many of the mechanistic steps are unknown and speculative. It is proposed that data from matrix isolation experiments involving the reactions of refractory materials (especially C, Si, and Fe atoms and clusters) with small molecules (mainly H2, H2O, CO, CO2) are particularly applicable to explaining mechanistic details of likely interstellar chemical reactions. In many cases, matrix isolation techniques are the sole method of studying such reactions; also in many cases, complexations and bond rearrangements yield molecules never before observed. The study of these reactions thus provides a logical basis for the mechanisms of interstellar reactions. A list of reactions is presented that would simulate interstellar chemical reactions. These reactions were studied using FTIR-matrix isolation techniques.

Discovery of interstellar circular polarization in the direction of the Crab Nebula.

NASA Technical Reports Server (NTRS)

Martin, P. G.; Illing, R.; Angel, J. R. P.

1972-01-01

A search in many small regions of the Crab Nebula has resulted in the detection of a small component of circular polarization. The variation of the sign and magnitude with position in the Nebula indicates that the polarization is of interstellar origin. On the basis of the polarity, strength, and colour dependence, it is concluded that the composition of the aligned grains causing this polarization is dielectric. Metallic particles are clearly ruled out. Some stars have also been observed with negative results.

THE INTERSTELLAR MAGNETIC FIELD CLOSE TO THE SUN. II

DOE Office of Scientific and Technical Information (OSTI.GOV)

Frisch, P. C.; Andersson, B-G; Berdyugin, A.

2012-12-01

The magnetic field in the local interstellar medium (ISM) provides a key indicator of the galactic environment of the Sun and influences the shape of the heliosphere. We have studied the interstellar magnetic field (ISMF) in the solar vicinity using polarized starlight for stars within 40 pc of the Sun and 90 Degree-Sign of the heliosphere nose. In Frisch et al. (Paper I), we developed a method for determining the local ISMF direction by finding the best match to a group of interstellar polarization position angles obtained toward nearby stars, based on the assumption that the polarization is parallel tomore » the ISMF. In this paper, we extend the analysis by utilizing weighted fits to the position angles and by including new observations acquired for this study. We find that the local ISMF is pointed toward the galactic coordinates l, b =47 Degree-Sign {+-} 20 Degree-Sign , 25 Degree-Sign {+-} 20 Degree-Sign . This direction is close to the direction of the ISMF that shapes the heliosphere, l, b =33 Degree-Sign {+-} 4 Degree-Sign , 55 Degree-Sign {+-} 4 Degree-Sign , as traced by the center of the 'Ribbon' of energetic neutral atoms discovered by the Interstellar Boundary Explorer (IBEX) mission. Both the magnetic field direction and the kinematics of the local ISM are consistent with a scenario where the local ISM is a fragment of the Loop I superbubble. A nearby ordered component of the local ISMF has been identified in the region l Almost-Equal-To 0 Degree-Sign {yields} 80 Degree-Sign and b Almost-Equal-To 0 Degree-Sign {yields} 30 Degree-Sign , where PlanetPol data show a distance-dependent increase of polarization strength. The ordered component extends to within 8 pc of the Sun and implies a weak curvature in the nearby ISMF of {approx}0.{sup 0}25 pc{sup -1}. This conclusion is conditioned on the small sample of stars available for defining this rotation. Variations from the ordered component suggest a turbulent component of {approx}23 Degree-Sign . The ordered component and standard relations between polarization, color excess, and H{sup o} column density predict a reasonable increase of N(H) with distance in the local ISM. The similarity of the ISMF directions traced by the polarizations, the IBEX Ribbon, and pulsars inside the Local Bubble in the third galactic quadrant suggest that the ISMF is relatively uniform over spatial scales of 8-200 pc and is more similar to interarm than spiral-arm magnetic fields. The ISMF direction from the polarization data is also consistent with small-scale spatial asymmetries detected in GeV-TeV cosmic rays with a galactic origin. The peculiar geometrical relation found earlier between the cosmic microwave background dipole moment, the heliosphere nose, and the ISMF direction is supported by this study. The interstellar radiation field at {approx}975 A does not appear to play a role in grain alignment for the low-density ISM studied here.« less

STOCHASTIC OPTICS: A SCATTERING MITIGATION FRAMEWORK FOR RADIO INTERFEROMETRIC IMAGING

DOE Office of Scientific and Technical Information (OSTI.GOV)

Johnson, Michael D., E-mail: mjohnson@cfa.harvard.edu

2016-12-10

Just as turbulence in the Earth’s atmosphere can severely limit the angular resolution of optical telescopes, turbulence in the ionized interstellar medium fundamentally limits the resolution of radio telescopes. We present a scattering mitigation framework for radio imaging with very long baseline interferometry (VLBI) that partially overcomes this limitation. Our framework, “stochastic optics,” derives from a simplification of strong interstellar scattering to separate small-scale (“diffractive”) effects from large-scale (“refractive”) effects, thereby separating deterministic and random contributions to the scattering. Stochastic optics extends traditional synthesis imaging by simultaneously reconstructing an unscattered image and its refractive perturbations. Its advantages over direct imagingmore » come from utilizing the many deterministic properties of the scattering—such as the time-averaged “blurring,” polarization independence, and the deterministic evolution in frequency and time—while still accounting for the stochastic image distortions on large scales. These distortions are identified in the image reconstructions through regularization by their time-averaged power spectrum. Using synthetic data, we show that this framework effectively removes the blurring from diffractive scattering while reducing the spurious image features from refractive scattering. Stochastic optics can provide significant improvements over existing scattering mitigation strategies and is especially promising for imaging the Galactic Center supermassive black hole, Sagittarius A*, with the Global mm-VLBI Array and with the Event Horizon Telescope.« less

The Formation of Racemic Amino Acids by UV Photolysis of Interstellar Ice Analogs

NASA Technical Reports Server (NTRS)

Bernstein, Max P.; Dworkin, Jason P.; Sandford, Scott A.; Cooper, George; Allamandola, Louis J.; DeVincenzi, Donald (Technical Monitor)

2001-01-01

Small biologically relevant organic molecules including the amino acids glycine, alanine, and marine were formed in the laboratory by the UV (Ultraviolet) photolysis of realistic interstellar ice analogs, composed primarily of H2O, and including CH3OH, NH3, and HCN, under interstellar conditions. N-formyl glycine, cycloserine (4-amino-3-isoxazolidinone), and glycerol were detected before hydrolysis, and glycine, racemic alanine, racemic marine, glycerol, ethanolamine, and glyceric acid were found after hydrolysis. This suggests that some meteoritic amino acids (and other molecules) may be the direct result of interstellar ice photochemistry, expanding the current paradigm that they formed by reactions in liquid water on meteorite parent bodies.

Small interstellar molecules and what they tell us

NASA Astrophysics Data System (ADS)

Neufeld, David A.

2018-06-01

Observations at ultraviolet, visible, infrared and radio wavelengths provide a wealth of information about the molecular inventory of the interstellar medium (ISM). Because of the different chemical pathways responsible for their formation and destruction, different molecules probe specific aspects of the interstellar environment. Carefully interpreted with the use of astrochemical models, they provide unique information of general astrophysical importance, yielding estimates of the cosmic ray density, the molecular fraction, the ultraviolet radiation field, and the dissipation of energy within the turbulent ISM. Laboratory experiments and quantum-mechanical calculations are essential both in providing the spectroscopic data needed to identify interstellar molecules and for elucidating the fundamental physical and chemical processes that must be included in astrochemical models.

Dust Spectroscopy and the Nature of Grains

NASA Technical Reports Server (NTRS)

Tielens, A. G. G. M.

2006-01-01

Ground-based, air-borne and space-based, infrared spectra of a wide variety of objects have revealed prominent absorption and emission features due to large molecules and small dust grains. Analysis of this data reveals a highly diverse interstellar and circumstellar grain inventory, including both amorphous materials and highly crystalline compounds (silicates and carbon). This diversity points towards a wide range of physical and chemical birthsites as well as a complex processing of these grains in the interstellar medium. In this talk, I will review the dust inventory contrasting and comparing both the interstellar and circumstellar reservoirs. The focus will be on the processes that play a role in the lifecycle of dust in the interstellar medium.

The molecular composition of dense interstellar clouds

NASA Technical Reports Server (NTRS)

Allen, M.; Robinson, G. W.

1977-01-01

Presented in this paper is an ab initio chemical model for dense interstellar clouds that incorporates 598 grain surface reactions, with small grains providing the reaction area. Gas-phase molecules are depleted through collisions with grains. The abundances of 372 chemical species are calculated as a function of time and are found to be of sufficient magnitude to explain most observations. Peak abundances are achieved on time scales of the order of 100,000 to 1 million years, depending on cloud density and kinetic temperature. The reaction rates for ion-molecule chemistry are approximately the same, indicating that surface and gas-phase chemistry may be coupled in certain regions. The composition of grain mantles is shown to be a function of grain radius. In certain grain-size ranges, large molecules containing two or more heavy atoms are more predominant than lighter 'ices' - H2O, NH3, and CH4. It is possible that absorption due to these large molecules in the mantle may contribute to the observed 3-micron band in astronomical spectra.

Probing the Spatial Distribution of the Interstellar Dust Medium by High Angular Resolution X-ray Halos of Point Sources

NASA Astrophysics Data System (ADS)

Xiang, Jingen

X-rays are absorbed and scattered by dust grains when they travel through the interstellar medium. The scattering within small angles results in an X-ray ``halo''. The halo properties are significantly affected by the energy of radiation, the optical depth of the scattering, the grain size distributions and compositions, and the spatial distribution of dust along the line of sight (LOS). Therefore analyzing the X-ray halo properties is an important tool to study the size distribution and spatial distribution of interstellar grains, which plays a central role in the astrophysical study of the interstellar medium, such as the thermodynamics and chemistry of the gas and the dynamics of star formation. With excellent angular resolution, good energy resolution and broad energy band, the Chandra ACIS is so far the best instrument for studying the X-ray halos. But the direct images of bright sources obtained with ACIS usually suffer from severe pileup which prevents us from obtaining the halos in small angles. We first improve the method proposed by Yao et al to resolve the X-ray dust scattering halos of point sources from the zeroth order data in CC-mode or the first order data in TE mode with Chandra HETG/ACIS. Using this method we re-analyze the Cygnus X-1 data observed with Chandra. Then we studied the X-ray dust scattering halos around 17 bright X-ray point sources using Chandra data. All sources were observed with the HETG/ACIS in CC-mode or TE-mode. Using the interstellar grain models of WD01 model and MRN model to fit the halo profiles, we get the hydrogen column densities and the spatial distributions of the scattering dust grains along the line of sights (LOS) to these sources. We find there is a good linear correlation not only between the scattering hydrogen column density from WD01 model and the one from MRN model, but also between N_{H} derived from spectral fits and the one derived from the grain models WD01 and MRN (except for GX 301-2 and Vela X-1): N_{H,WD01} = (0.720±0.009) × N_{H,abs} + (0.051±0.013) and N_{H, MRN} = (1.156±0.016) × N_{H,abs} + (0.062±0.024) in the units 10^{22} cm^{-2}. Then the correlation between FHI and N_{H} is obtained. Both WD01 model and MRN model fits show that the scattering dust density very close to these sources is much higher than the normal interstellar medium and we consider it is the evidence of molecular clouds around these X-ray binaries. We also find that there is the linear correlation between the effective distance through the galactic dust layer and hydrogen scattering olumn density N_{H} excluding the one in x=0.99-1.0 but the correlation does not exist between he effective distance and the N_{H} in x=0.99-1.0. It shows that the dust nearby the X-ray sources is not the dust from galactic disk. Then we estimate the structure and density of the stellar wind around the special X-ray pulsars Vela X-1 and GX 301-2. Finally we discuss the possibility of probing the three dimensional structure of the interstellar using the X-ray halos of the transient sources, probing the spatial distributions of interstellar dust medium nearby the point sources, even the structure of the stellar winds using higher angular resolution X-ray dust scattering halos and testing the model that the black hole can be formed from the direct collapse of a massive star without supernova using the statistical distribution of the dust density nearby the X-ray binaries.

Simulating the interaction of the heliosphere with the local interstellar medium: MHD results from a finite volume approach, first bidimensional results

NASA Technical Reports Server (NTRS)

Chanteur, G.; Khanfir, R.

1995-01-01

We have designed a full compressible MHD code working on unstructured meshes in order to be able to compute accurately sharp structures embedded in large scale simulations. The code is based on a finite volume method making use of a kinetic flux splitting. A bidimensional version of the code has been used to simulate the interaction of a moving interstellar medium, magnetized or unmagnetized with a rotating and magnetized heliopspheric plasma source. Being aware that these computations are not realistic due to the restriction to two dimensions, we present it to demonstrate the ability of this new code to handle this problem. An axisymetric version, now under development, will be operational in a few months. Ultimately we plan to run a full 3d version.

Aluminum Foils of the Stardust Interstellar Collector: The Challenge of Recognizing Micrometer-sized Impact Craters made by Interstellar Grains

NASA Technical Reports Server (NTRS)

Kearsley, A. T.; Westphal, A. J.; Burchell, M. J.; Zolensky, Michael E.

2008-01-01

Preliminary Examination (PE) of the Stardust cometary collector revealed material embedded in aerogel and on aluminium (Al) foil. Large numbers of sub-micrometer impact craters gave size, structural and compositional information. With experience of finding and analyzing the picogram to nanogram mass remains of cometary particles, are we now ready for PE of the Interstellar (IS) collector? Possible interstellar particle (ISP) tracks in the aerogel are being identified by the stardust@home team. We are now assessing challenges facing PE of Al foils from the interstellar collector.

Effects of multiple-scale driving on turbulence statistics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yoo, Hyunju; Cho, Jungyeon, E-mail: hyunju527@gmail.com, E-mail: jcho@cnu.ac.kr

2014-01-01

Turbulence is ubiquitous in astrophysical fluids such as the interstellar medium and the intracluster medium. In turbulence studies, it is customary to assume that fluid is driven on a single scale. However, in astrophysical fluids, there can be many different driving mechanisms that act on different scales. If there are multiple energy-injection scales, the process of energy cascade and turbulence dynamo will be different compared with the case of the single energy-injection scale. In this work, we perform three-dimensional incompressible/compressible magnetohydrodynamic turbulence simulations. We drive turbulence in Fourier space in two wavenumber ranges, 2≤k≤√12 (large scale) and 15 ≲ kmore » ≲ 26 (small scale). We inject different amount of energy in each range by changing the amplitude of forcing in the range. We present the time evolution of the kinetic and magnetic energy densities and discuss the turbulence dynamo in the presence of energy injections at two scales. We show how kinetic, magnetic, and density spectra are affected by the two-scale energy injections and we discuss the observational implications. In the case ε {sub L} < ε {sub S}, where ε {sub L} and ε {sub S} are energy-injection rates at the large and small scales, respectively, our results show that even a tiny amount of large-scale energy injection can significantly change the properties of turbulence. On the other hand, when ε {sub L} ≳ ε {sub S}, the small-scale driving does not influence the turbulence statistics much unless ε {sub L} ∼ ε {sub S}.« less

Silicon chemistry in interstellar clouds

NASA Technical Reports Server (NTRS)

Langer, William D.; Glassgold, A. E.

1989-01-01

Interstellar SiO was discovered shortly after CO but it has been detected mainly in high density and high temperature regions associated with outflow sources. A new model of interstellar silicon chemistry that explains the lack of SiO detections in cold clouds is presented which contains an exponential temperature dependence for the SiO abundance. A key aspect of the model is the sensitivity of SiO production by neutral silicon reactions to density and temperature, which arises from the dependence of the rate coefficients on the population of the excited fine structure levels of the silicon atom. This effect was originally pointed out in the context of neutral reactions of carbon and oxygen by Graff, who noted that the leading term in neutral atom-molecule interactions involves the quadrupole moment of the atom. Similar to the case of carbon, the requirement that Si has a quadrupole moment requires population of the J = 1 level, which lies 111K above the J = 0 ground state and has a critical density n(cr) equal to or greater than 10(6)/cu cm. The SiO abundance then has a temperature dependence proportional to exp(-111/T) and a quadratic density dependence for n less than n(cr). As part of the explanation of the lack of SiO detections at low temperatures and densities, this model also emphasizes the small efficiencies of the production routes and the correspondingly long times needed to reach equilibrium. Measurements of the abundance of SiO, in conjunction with theory, can provide information on the physical properties of interstellar clouds such as the abundances of oxygen bearing molecules and the depletion of interstellar silicon.

The Local Bubble: a magnetic veil to our Galaxy

NASA Astrophysics Data System (ADS)

Alves, M. I. R.; Boulanger, F.; Ferrière, K.; Montier, L.

2018-04-01

The magnetic field in the local interstellar medium does not follow the large-scale Galactic magnetic field. The local magnetic field has probably been distorted by the Local Bubble, a cavity of hot ionized gas extending all around the Sun and surrounded by a shell of cold neutral gas and dust. However, so far no conclusive association between the local magnetic field and the Local Bubble has been established. Here we develop an analytical model for the magnetic field in the shell of the Local Bubble, which we represent as an inclined spheroid, off-centred from the Sun. We fit the model to Planck dust polarized emission observations within 30° of the Galactic poles. We find a solution that is consistent with a highly deformed magnetic field, with significantly different directions towards the north and south Galactic poles. This work sets a methodological framework for modelling the three-dimensional (3D) structure of the magnetic field in the local interstellar medium, which is a most awaited input for large-scale Galactic magnetic field models.

Variable interstellar absorption lines in young stellar aggregates

NASA Astrophysics Data System (ADS)

Krełowski, J.; Strobel, A.; Vješnica, S.; Melekh, D.; Bondar, A.

2018-06-01

The variability of interstellar atomic lines, sporadically reported in the astronomical literature, has been confirmed both in the case of the nearby hot star δ Ori and the very young and violent star-forming region η Carinae, using high-resolution echelle spectra. The presented variability concerns the intensities and profiles of Na I, K I and Ca II. The time-scale of the above-mentioned variations clearly suggests very local phenomena as their cause. It is important to say that not all interstellar lines vary in unison.

Primordial black holes as seeds of magnetic fields in the universe

NASA Astrophysics Data System (ADS)

Safarzadeh, Mohammadtaher

2018-06-01

Although it is assumed that magnetic fields in accretion disks are dragged from the interstellar medium, the idea is likely not applicable to primordial black holes (PBHs) formed in the early universe. Here we show that magnetic fields can be generated in initially unmagnetized accretion disks around PBHs through the Biermann battery mechanism, and therefore provide the small scale seeds of magnetic field in the universe. The radial temperature and vertical density profiles of these disks provide the necessary conditions for the battery to operate naturally. The generated seed fields have a toroidal structure with opposite sign in the upper and lower half of the disk. In the case of a thin accretion disk around a rotating PBH, the field generation rate increases with increasing PBH spin. At a fixed r/risco, where r is the radial distance from the PBH and risco is the radius of the innermost stable circular orbit, the battery scales as M-9/4, where M is the PBH's mass. The very weak dependency of the battery on accretion rate, makes this mechanism a viable candidate to provide seed fields in an initially unmagnetized accretion disk, following which the magnetorotational instability could take over.

Aliphatic Hydrocarbon Content of Interstellar Dust

NASA Astrophysics Data System (ADS)

Günay, B.; Schmidt, T. W.; Burton, M. G.; Afşar, M.; Krechkivska, O.; Nauta, K.; Kable, S. H.; Rawal, A.

2018-06-01

There is considerable uncertainty as to the amount of carbon incorporated in interstellar dust. The aliphatic component of the carbonaceous dust is of particular interest because it produces a significant 3.4 μm absorption feature when viewed against a background radiation source. The optical depth of the 3.4 μm absorption feature is related to the number of aliphatic carbon C-H bonds along the line of sight. It is possible to estimate the column density of carbon locked up in the aliphatic hydrocarbon component of interstellar dust from quantitative analysis of the 3.4 μm interstellar absorption feature providing that the absorption coefficient of aliphatic hydrocarbons incorporated in the interstellar dust is known. We report laboratory analogues of interstellar dust by experimentally mimicking interstellar/circumstellar conditions. The resultant spectra of these dust analogues closely match those from astronomical observations. Measurements of the absorption coefficient of aliphatic hydrocarbons incorporated in the analogues were carried out by a procedure combining FTIR and 13C NMR spectroscopies. The absorption coefficients obtained for both interstellar analogues were found to be in close agreement (4.76(8) × 10-18 cm group-1 and 4.69(14) × 10-18 cm group-1), less than half those obtained in studies using small aliphatic molecules. The results thus obtained permit direct calibration of the astronomical observations, providing rigorous estimates of the amount of aliphatic carbon in the interstellar medium.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bzowski, M.; Kubiak, M. A.; Sokol, J. M.

Because of its high ionization potential and weak interaction with hydrogen, neutral interstellar helium (NISHe) is almost unaffected at the heliospheric interface with the interstellar medium and freely enters the solar system. This second most abundant species provides some of the best information on the characteristics of the interstellar gas in the local interstellar cloud. The Interstellar Boundary Explorer (IBEX) is the second mission to directly detect NISHe. We present a comparison between recent IBEX NISHe observations and simulations carried out using a well-tested quantitative simulation code. Simulation and observation results compare well for times when measured fluxes are dominatedmore » by NISHe (and contributions from other species are small). Differences between simulations and observations indicate a previously undetected secondary population of neutral helium, likely produced by interaction of interstellar helium with plasma in the outer heliosheath. Interstellar neutral parameters are statistically different from previous in situ results obtained mostly from the GAS/Ulysses experiment, but they do agree with the local interstellar flow vector obtained from studies of interstellar absorption: the newly established flow direction is ecliptic longitude 79.{sup 0}2, latitude -5.{sup 0}1, the velocity is {approx}22.8 km s{sup -1}, and the temperature is 6200 K. These new results imply a markedly lower absolute velocity of the gas and thus significantly lower dynamic pressure on the boundaries of the heliosphere and different orientation of the Hydrogen Deflection Plane compared to prior results from Ulysses. A different orientation of this plane also suggests a new geometry of the interstellar magnetic field, and the lower dynamic pressure calls for a compensation by other components of the pressure balance, most likely a higher density of interstellar plasma and strength of interstellar magnetic field.« less

Laser Boost of a Small Interstellar Ram Jet to Obtain Operational Velocity. Implications for the DM Rocket/Ram Jet Model

NASA Astrophysics Data System (ADS)

Walcott Beckwith, Andrew

2010-05-01

In other conference research papers, Beckwith obtained a maximum DM mass/energy value of up to 5 TeV, as opposed to 400 GeV for DM, which may mean more convertible power for a dark matter ram jet. The consequences are from assuming that axions are CDM, and KK gravitons are for WDM, then ρWarm-Dark-Matter would dominate not only structure formation in early universe formation, but would also influence the viability of the DM ram jet applications for interstellar travel. The increase in convertible DM mass makes the ram jet a conceivable option. This paper in addition to describing the scientific issues leading to that 5 TeV mass for DM also what are necessary and sufficient laser boost systems which would permit a ram net to become operational.

The Centauri project: Manned interstellar travel

NASA Technical Reports Server (NTRS)

Ciesla, Thomas M.

1990-01-01

The development of antimatter engines for spacecraft propulsion will allow man to expand to the nearest stellar neighbors such as the Alpha Centuri system. Compared to chemically powered rockets like the Apollo mission class which would take 50,000 years to reach the Centauri system, antimatter propulsion would reduce one way trip time to 30 years or less. The challenges encountered by manned interstellar travel are formidable. The spacecraft must be a combination of sublight speed transportation system and a traveling microplanet serving an expanding population. As the population expands from the initial 100 people to approximately 300, the terraformed asteroid, enclosed by a man-made shell will allow for expansion over its surface in the fashion of a small terrestrial town. All aspects of human life - birth; death; physical, emotional, and educational needs; and government and law must be met by the structure, systems, and institutions on-board.

Decoding IR Spectra of Cosmic Ices and Organics in the Laboratory

NASA Technical Reports Server (NTRS)

Allamandola, Louis J.

2006-01-01

Tremendous strides have been made in our understanding of interstellar material over the past twenty-five years thanks to significant developments in observational IR astronomy and dedicated laboratory experiments. Twenty-five years ago the composition of interstellar dust was largely guessed at. Today the composition of interstellar dust is reasonably well understood. In the diffuse interstellar medium (ISM) the dust population is mainly comprised of small grains of silicates and amorphous carbon. In dark molecular clouds, the birthplace of stars and planets, these cold refractory dust particles are coated with mixed molecular ices whose composition is reasonably well constrained. Lastly, the signature of carbon-rich polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by early interstellar chemistry standards, is widespread throughout the Universe. This extraordinary progress has been made possible by the close collaboration of laboratory experimentalists and theoreticians with IR astronomers using groundbased, air-borne, and orbiting telescopes.

Radio Wave Propagation through a Medium Containing Electron Density Fluctuations Described by an Anisotropic Goldreich-Sridhar Spectrum

NASA Astrophysics Data System (ADS)

Chandran, B. D. G.; Backer, D. C.

2002-09-01

We study the propagation of radio waves through a medium possessing density fluctuations that are elongated along the ambient magnetic field and described by an anisotropic Goldreich-Sridhar power spectrum. We derive general formulae for the wave phase structure function Dφ, visibility, angular broadening, diffraction pattern length scales, and scintillation timescale for arbitrary distributions of turbulence along the line of sight and specialize these formulae to idealized cases. In general, Dφ~(δr)5/3 when the baseline δr is in the inertial range of the turbulent density spectrum, and Dφ~(δr)2 when δr is in the dissipation range, just as for an isotropic Kolmogorov spectrum of fluctuations. When the density structures that dominate the scattering have an axial ratio R>>1 (typically R~103), the axial ratio of the broadened image of a point source in the standard Markov approximation is at most ~R1/2, and this maximum value is obtained in the unrealistic case that the scattering medium is confined to a thin screen in which the magnetic field has a single direction. If the projection of the magnetic field within the screen onto the plane of the sky rotates through an angle Δψ along the line of sight from one side of the screen to the other, and if R-1/2<<Δψ<<1, then the axial ratio of the resulting broadened image of a point source is 2(8/3)3/5/Δψ~=3.6/Δψ. The error in this formula increases with Δψ but reaches only ~15% when Δψ=π. This indicates that a moderate amount of variation in the direction of the magnetic field along the line of sight dramatically decreases the anisotropy of a broadened image. When R>>1, the observed anisotropy will in general be determined by the degree of variation of the field direction along the sight line and not by the degree of density anisotropy. Although this makes it difficult to determine observationally the degree of anisotropy in interstellar density fluctuations, observed anisotropies in broadened images provide general support for anisotropic models of interstellar turbulence. Regions in which the angle γ between the magnetic field and line of sight is small cause enhanced scattering due to the increased coherence of density structures along the line of sight. In the exceedingly rare and probably unrealized case that scattering is dominated by regions in which γ<~(δr/l)1/3, where l is the outer scale (stirring scale) of the turbulence, Dφ~(δr)4/3 for δr in the inertial range. In a companion paper (Backer & Chandran) we discuss the semiannual modulation in the scintillation time of a nearby pulsar for which the field direction variation along the line of sight is expected to be moderately small.

The fuelling of active galactic nuclei

NASA Technical Reports Server (NTRS)

Shlosman, Isaac; Begelman, Mitchell C.; Frank, Julian

1990-01-01

Accretion mechanisms for powering the central engines of active galactic nuclei (AGN) and possible sources of fuel are reviewed. It is a argued that the interstellar matter in the main body of the host galaxy is channeled toward the center, and the problem of angular momentum transport is addressed. Thin accretion disks are not a viable means of delivering fuel to luminous AGN on scales much larger than a parsec because of the long inflow time and effects of self-gravity. There are also serious obstacles to maintaining and regulating geometrically thick, hot accretion flows. The role of nonaxisymmetric perturbations of the gravitational potential on galactic scales and their triggers is emphasized. A unified model is outlined for fueling AGN, in which the inflow on large scales is driven by gravitational torques, and on small scales forms a mildly self-gravitating disk of clouds with inflow driven by magnetic torques or cloud-cloud collisions.

An astrosphere around the blue supergiant κ Cas: possible explanation of its filamentary structure

NASA Astrophysics Data System (ADS)

Katushkina, O. A.; Alexashov, D. B.; Gvaramadze, V. V.; Izmodenov, V. V.

2018-01-01

High-resolution mid-infrared observations carried out by the Spitzer Space Telescope allowed one to resolve the fine structure of many astrospheres. In particular, they showed that the astrosphere around the B0.7 Ia star κ Cas (HD 2905) has a clear-cut arc structure with numerous cirrus-like filaments beyond it. Previously, we suggested a physical mechanism for the formation of such filamentary structures. Namely, we showed theoretically that they might represent the non-monotonic spatial distribution of the interstellar dust in astrospheres (viewed as filaments) caused by interaction of the dust grains with the interstellar magnetic field disturbed in the astrosphere due to colliding of the stellar and interstellar winds. In this paper, we invoke this mechanism to explain the structure of the astrosphere around κ Cas. We performed 3D magnetohydrodynamic modelling of the astrosphere for realistic parameters of the stellar wind and space velocity. The dust dynamics and the density distribution in the astrosphere were calculated in the framework of a kinetic model. It is found that the model results with the classical MRN (Mathis, Rumpl & Nordsieck 1977) size distribution of dust in the interstellar medium do not match the observations, and that the observed filamentary structure of the astrosphere can be reproduced only if the dust is composed mainly of big (μm-sized) grains. Comparison of the model results with observations allowed us to estimate parameters (number density and magnetic field strength) of the surrounding interstellar medium.

Variations in the Peak Position of the 6.2 micron Interstellar Emission Feature: A Tracer of N in the Interstellar Polycyclic Aromatic Hydrocarbon Population

NASA Technical Reports Server (NTRS)

Hudgins, Douglas M.; Bauschlicher, Charles W.; Allamandola, L. J.

2005-01-01

This paper presents the results of an investigation of the molecular characteristics that underlie the observed peak position and profile of the nominal 6.2 micron interstellar emission band generally attributed to the CC stretching vibrations of polycyclic aromatic hydrocarbons (PAHs). It begins with a summary of recent experimental and theoretical studies ofthe spectroscopic properties of large (>30 carbon atoms) PAH cations as they relate to this aspect of the astrophysical problem. It then continues with an examination of the spectroscopic properties of a number of PAH variants within the context of the interstellar 6.2 micron emission, beginning with a class of compounds known as polycyclic aromatic nitrogen heterocycles (PANHs; PAHs with one or more nitrogen atoms substituted into their carbon skeleton). In this regard, we summarize the results of recent relevant experimental studies involving a limited set of small PANHs and their cations and then report the results of a comprehensive computational study that extends that work to larger PANH cations including many nitrogen-substituted variants of coronene(+) (C24H12(+)), ovalene(+) (C32H14(+)), circumcoronene(+) (C54H18(+)), and circum-circumcoronene(+) (C96H24(+)). Finally, we report the results of more focused computational studies of selected representatives from a number of other classes of PAH variants that share one or more of the key attributes of the PANH species studied. These alternative classes of PAH variants include (1) oxygen- and silicon-substituted PAH cations; (2) PAH-metal ion complexes (metallocenes) involving the cosmically abundant elements magnesium and iron; and (3) large, asymmetric PAH cations. Overall, the studies reported here demonstrate that increasing PAH size alone is insuEcient to account for the position of the shortest wavelength interstellar 6.2 micron emission bands, as had been suggested by earlier studies. On the other hand, this work reveals that substitution of one or more nitrogen atoms within the interior of the carbon skeleton of a PAH cation induces a significant blueshift in the position of the dominant CC stretching feature of these compounds that is suf6cient to account for the position of the interstellar bands. Subsequent studies of the effects of substitution by other heteroatoms (O and Si), metal ion complexation (Fe(+), Mg(+), and Mg(2+)), and molecular symmetry variation-all of which fail to reproduce the blueshift observed in the PANH cations-indicate that N appears to be unique in its ability to accommodate the position of the interstellar 6.2 micron bands while simultaneously satisfying the other constraints of the astrophysical problem. This result implies that the peak position of the interstellar feature near 6.2 micron traces the degree of nitrogen substitution in the population, that most of the PAHs responsible for the interstellar IR emission features incorporate nitrogen within their aromatic networks, and that a lower limit of 1%-2% of the cosmic nitrogen is sequestered within the interstellar PAH population. Finally, in view of the ubiquity and abundance of interstellar PAHs and the permanent dipoles and distinctive electronic structures of these nitrogen-substituted variants, this work impacts a wide range of observational phenomena outside of the infrared region of the spectrum including the forest of unidentified molecular rotational features and the anomalous Galactic foreground emission in the microwave, and the diffuse interstellar bands (DIBs) and other structure in the interstellar extinction curve in the ulhviolet/visible. These astrophysical ramifications are discussed, and the dipole moments and rotational constants are tabulated to facilitate further investigations of the astrophysical role of nitrogen-substituted aromatic compounds.

An Essay on Interactive Investigations of the Zeeman Effect in the Interstellar Medium

ERIC Educational Resources Information Center

Woolsey, Lauren

2015-01-01

The paper presents an interactive module created through the Wolfram Demonstrations Project that visualizes the Zeeman effect for the small magnetic field strengths present in the interstellar medium. The paper provides an overview of spectral lines and a few examples of strong and weak Zeeman splitting before discussing the module in depth.…

The Fermi paradox: An approach based on percolation theory

NASA Technical Reports Server (NTRS)

Landis, Geoffrey A.

1993-01-01

If even a very small fraction of the hundred billion stars in the galaxy are home to technological civilizations which colonize over interstellar distances, the entire galaxy could be completely colonized in a few million years. The absence of such extraterrestrial civilizations visiting Earth is the Fermi paradox. A model for interstellar colonization is proposed using the assumption that there is a maximum distance over which direct interstellar colonization is feasible. Due to the time lag involved in interstellar communications, it is assumed that an interstellar colony will rapidly develop a culture independent of the civilization that originally settled it. Any given colony will have a probability P of developing a colonizing civilization, and a probability (1-P) that it will develop a non-colonizing civilization. These assumptions lead to the colonization of the galaxy occuring as a percolation problem. In a percolation problem, there will be a critical value of percolation probability, P(sub c). For P less than P(sub c), colonization will always terminate after a finite number of colonies. Growth will occur in 'clusters', with the outside of each cluster consisting of non-colonizing civilizations. For P greater than P(sub c), small uncolonized voids will exist, bounded by non-colonizing civilizations. For P approximately = to P(sub c), arbitrarily large filled regions exist, and also arbitrarily large empty regions.

Interstellar polycyclic aromatic hydrocarbons - The infrared emission bands, the excitation/emission mechanism, and the astrophysical implications

NASA Technical Reports Server (NTRS)

Allamandola, L. J.; Tielens, G. G. M.; Barker, J. R.

1989-01-01

A comprehensive study of the PAH hypothesis is presented, including the interstellar, IR spectral features which have been attributed to emission from highly vibrationally excited PAHs. Spectroscopic and IR emission features are discussed in detail. A method for calculating the IR fluorescence spectrum from a vibrationally excited molecule is described. Analysis of interstellar spectrum suggests that the PAHs which dominate the IR spectra contain between 20 and 40 C atoms. The results are compared with results from a thermal approximation. It is found that, for high levels of vibrational excitation and emission from low-frequency modes, the two methods produce similar results. Also, consideration is given to the relationship between PAH molecules and amorphous C particles, the most likely interstellar PAH molecular structures, the spectroscopic structure produced by PAHs and PAH-related materials in the UV portion of the interstellar extinction curve, and the influence of PAH charge on the UV, visible, and IR regions.

Interstellar Propulsion Research Within NASA

NASA Technical Reports Server (NTRS)

Johnson, Les; Cook, Stephen (Technical Monitor)

2001-01-01

NASA is actively conducting advanced propulsion research and technology development in various in-space transportation technologies with potential application to interstellar missions and precursors. Within the last few years, interest in the scientific community in interstellar missions as well as outer heliospheric missions, which could function as interstellar precursor missions, has increased. A mission definition team was charted by NASA to define such a precursor, The Interstellar Probe, which resulted in a prioritization of relatively near-term transportation technologies to support its potential implementation. In addition, the goal of finding and ultimately imaging extra solar planets has raised the issue of our complete inability to mount an expedition to such as planet, should one be found. Even contemplating such a mission with today's technology is a stretch of the imagination. However, there are several propulsion concepts, based on known physics, that have promise to enable interstellar exploration in the future. NASA is making small, incremental investments in some key advanced propulsion technologies in an effort to advance their state-of-the-art in support potential future mission needs. These technologies, and their relative maturity, are described.

Tentative Identification of Interstellar Dust in the Magnetic Wall of the Heliosphere

NASA Astrophysics Data System (ADS)

Frisch, Priscilla C.

2005-10-01

Observations of the weak polarization of light from nearby stars, reported by Tinbergen, are consistent with polarization by small (radius <0.14 μm), interstellar dust grains entrained in the magnetic wall of the heliosphere. The region of maximum polarization is toward ecliptic coordinates (λ, β)~(295deg, 0deg), corresponding to (l, b) = (20°, -21°). The direction of maximum polarization is offset along the ecliptic longitude by ~35° from the nose of the heliosphere and extends to low ecliptic latitudes. An offset is also seen between the region with the best-aligned dust grains, λ~281deg-330deg, and the upwind direction of the undeflected large grains, λ~259deg, β~+8deg, which are observed by Ulysses and Galileo to be flowing into the heliosphere. In the aligned-grain region, the strength of polarization anticorrelates with ecliptic latitude, indicating that the magnetic wall is predominantly at negative ecliptic latitudes. An extension of the magnetic wall to β<0deg, formed by the interstellar magnetic field BIS draped over the heliosphere, is consistent with predictions by Linde (1998). A consistent interpretation follows if the maximum-polarization region traces the heliosphere magnetic wall in a direction approximately perpendicular to BIS, while the region of best-aligned dust samples the region where BIS drapes smoothly over the heliosphere with maximum compression. These data are consistent with BIS being tilted by 60° with respect to the ecliptic plane and parallel to the Galactic plane. Interstellar dust grains captured in the heliosheath may also introduce a weak, but important, large-scale contaminant for the cosmic microwave background signal with a symmetry consistent with the relative tilts of BIS and the ecliptic.

Magnetic Fields in the Interstellar Medium

NASA Astrophysics Data System (ADS)

Clark, Susan

2017-01-01

The Milky Way is magnetized. Invisible magnetic fields thread the Galaxy on all scales and play a vital but still poorly understood role in regulating flows of gas in the interstellar medium and the formation of stars. I will present highlights from my thesis work on magnetic fields in the diffuse interstellar gas and in accretion disks. At high Galactic latitudes, diffuse neutral hydrogen is organized into an intricate network of slender linear features. I will show that these neutral hydrogen “fibers” are extremely well aligned with the ambient magnetic field as traced by both starlight polarization (Clark et al. 2014) and Planck 353 GHz polarized dust emission (Clark et al. 2015). The structure of the neutral interstellar medium is more tightly coupled to the magnetic field than previously known. Because the orientation of neutral hydrogen is an independent predictor of the local dust polarization angle, our work provides a new tool in the search for inflationary gravitational wave B-mode polarization in the cosmic microwave background, which is currently limited by dust foreground contamination. Magnetic fields also drive accretion in astrophysical disks via the magnetorotational instability (MRI). I analytically derive the behavior of this instability in the weakly nonlinear regime and show that the saturated state of the instability depends on the geometry of the background magnetic field. The analytical model describes the behavior of the MRI in a Taylor-Couette flow, a set-up used by experimentalists in the ongoing quest to observe MRI in the laboratory (Clark & Oishi 2016a, 2016b).

Massive runaway stars in the Small Magellanic Cloud

NASA Astrophysics Data System (ADS)

Gvaramadze, V. V.; Pflamm-Altenburg, J.; Kroupa, P.

2011-01-01

Using archival Spitzer Space Telescope data, we identified for the first time a dozen runaway OB stars in the Small Magellanic Cloud (SMC) through the detection of their bow shocks. The geometry of detected bow shocks allows us to infer the direction of motion of the associated stars and to determine their possible parent clusters and associations. One of the identified runaway stars, AzV 471, was already known as a high-velocity star on the basis of its high peculiar radial velocity, which is offset by ≃ 40 km s-1 from the local systemic velocity. We discuss implications of our findings for the problem of the origin of field OB stars. Several of the bow shock-producing stars are found in the confines of associations, suggesting that these may be “alien” stars contributing to the age spread observed for some young stellar systems. We also report the discovery of a kidney-shaped nebula attached to the early WN-type star SMC-WR3 (AzV 60a). We interpreted this nebula as an interstellar structure created owing to the interaction between the stellar wind and the ambient interstellar medium.

Influence of galactic arm scale dynamics on the molecular composition of the cold and dense ISM. I. Observed abundance gradients in dense clouds

NASA Astrophysics Data System (ADS)

Ruaud, M.; Wakelam, V.; Gratier, P.; Bonnell, I. A.

2018-04-01

Aim. We study the effect of large scale dynamics on the molecular composition of the dense interstellar medium during the transition between diffuse to dense clouds. Methods: We followed the formation of dense clouds (on sub-parsec scales) through the dynamics of the interstellar medium at galactic scales. We used results from smoothed particle hydrodynamics (SPH) simulations from which we extracted physical parameters that are used as inputs for our full gas-grain chemical model. In these simulations, the evolution of the interstellar matter is followed for 50 Myr. The warm low-density interstellar medium gas flows into spiral arms where orbit crowding produces the shock formation of dense clouds, which are held together temporarily by the external pressure. Results: We show that depending on the physical history of each SPH particle, the molecular composition of the modeled dense clouds presents a high dispersion in the computed abundances even if the local physical properties are similar. We find that carbon chains are the most affected species and show that these differences are directly connected to differences in (1) the electronic fraction, (2) the C/O ratio, and (3) the local physical conditions. We argue that differences in the dynamical evolution of the gas that formed dense clouds could account for the molecular diversity observed between and within these clouds. Conclusions: This study shows the importance of past physical conditions in establishing the chemical composition of the dense medium.

PSR B0329+54: Statistics of Substructure Discovered within the Scattering Disk on RadioAstron Baselines of up to 235,000 km

NASA Astrophysics Data System (ADS)

Gwinn, C. R.; Popov, M. V.; Bartel, N.; Andrianov, A. S.; Johnson, M. D.; Joshi, B. C.; Kardashev, N. S.; Karuppusamy, R.; Kovalev, Y. Y.; Kramer, M.; Rudnitskii, A. G.; Safutdinov, E. R.; Shishov, V. I.; Smirnova, T. V.; Soglasnov, V. A.; Steinmassl, S. F.; Zensus, J. A.; Zhuravlev, V. I.

2016-05-01

We discovered fine-scale structure within the scattering disk of PSR B0329+54 in observations with the RadioAstron ground-space radio interferometer. Here we describe this phenomenon, characterize it with averages and correlation functions, and interpret it as the result of decorrelation of the impulse-response function of interstellar scattering between the widely separated antennas. This instrument included the 10 m Space Radio Telescope, the 110 m Green Bank Telescope, the 14 × 25 m Westerbork Synthesis Radio Telescope, and the 64 m Kalyazin Radio Telescope. The observations were performed at 324 MHz on baselines of up to 235,000 km in 2012 November and 2014 January. In the delay domain, on long baselines the interferometric visibility consists of many discrete spikes within a limited range of delays. On short baselines it consists of a sharp spike surrounded by lower spikes. The average envelope of correlations of the visibility function shows two exponential scales, with characteristic delays of {τ }1=4.1+/- 0.3 μ {{s}} and {τ }2=23+/- 3 μ {{s}}, indicating the presence of two scales of scattering in the interstellar medium. These two scales are present in the pulse-broadening function. The longer scale contains 0.38 times the scattered power of the shorter one. We suggest that the longer tail arises from highly scattered paths, possibly from anisotropic scattering or from substructure at large angles.

Massive stars: privileged sources of cosmic-rays for interstellar astrochemistry

NASA Astrophysics Data System (ADS)

De Becker, M.

2015-01-01

Massive stars can be considered as crucial engines for interstellar physics. They are indeed the main providers of UV radiation field, and constitute a substantial source of chemical enrichment. On their evolution time-scale (at most about 10 Myr), they typically stay close to their formation site, i.e. close to molecular clouds very rich in interstellar molecules. These stellar objects have also the property to be involved in particle acceleration processes leading to the production of high energy charged particles (cosmic-rays). After rejection in the interstellar medium, these particles will play a substantial role in processes such as those simulated in various facilities dedicated to experimental astrochemistry. This short contribution intends to put these particles, crucial for astrochemistry, in their adequate astrophysical context.

A reanalysis of the HCO(+)/HOC(+) abundance ratio in dense interstellar clouds

NASA Technical Reports Server (NTRS)

Jarrold, M. F.; Bowers, M. T.; Defrees, D. J.; Mclean, A. D.; Herbst, E.

1986-01-01

New theoretical and experimental results have prompted a reinvestigation of the HCO(+)/HOC(+) abundance ratio in dense interstellar clouds. These results pertain principally but not exclusively to the reaction between HOC(+) and H2, which was previously calculated by DeFrees et al. (1984) to possess a large activation energy barrier. New calculations, reported here, indicate that this activation energy barrier is quite small and may well be zero. In addition, experimental results at higher energy and temperature indicate strongly that the reaction proceeds efficiently at interstellar temperatures. If HOC(+) does indeed react efficiently with H2 in interstellar clouds, the calculated HCO(+)/HOC(+) abundance ratio rises to a substantially greater value under standard dense cloud conditions than is deduced via the tentative observation of HOC(+) in Sgr B2.

Observations of the interstellar gas with the Copernicus satellite

NASA Technical Reports Server (NTRS)

Morton, D. C.

1975-01-01

Results are reviewed for Copernicus far-UV measurements of the absorption lines of H I, D I, H2, and heavier elements in the interstellar gas. Column densities along several lines of sight, as estimated from Ly-alpha absorption-line profiles, confirm that wide differences in the gas density are present in various directions. The measurement of interstellar D I implies an open universe unless alternate sources for this nuclide are found. Analysis of reddened stars for which the line of sight passes through one or more interstellar clouds indicates a depletion of several heavy elements in the gas. It is suggested that the depleted elements may be present in grains rather than molecules and that the intercloud medium may consist primarily of H II with a few small H I clouds.

A Census of Large-scale (≥10 PC), Velocity-coherent, Dense Filaments in the Northern Galactic Plane: Automated Identification Using Minimum Spanning Tree

NASA Astrophysics Data System (ADS)

Wang, Ke; Testi, Leonardo; Burkert, Andreas; Walmsley, C. Malcolm; Beuther, Henrik; Henning, Thomas

2016-09-01

Large-scale gaseous filaments with lengths up to the order of 100 pc are on the upper end of the filamentary hierarchy of the Galactic interstellar medium (ISM). Their association with respect to the Galactic structure and their role in Galactic star formation are of great interest from both an observational and theoretical point of view. Previous “by-eye” searches, combined together, have started to uncover the Galactic distribution of large filaments, yet inherent bias and small sample size limit conclusive statistical results from being drawn. Here, we present (1) a new, automated method for identifying large-scale velocity-coherent dense filaments, and (2) the first statistics and the Galactic distribution of these filaments. We use a customized minimum spanning tree algorithm to identify filaments by connecting voxels in the position-position-velocity space, using the Bolocam Galactic Plane Survey spectroscopic catalog. In the range of 7\\buildrel{\\circ}\\over{.} 5≤slant l≤slant 194^\\circ , we have identified 54 large-scale filaments and derived mass (˜ {10}3{--}{10}5 {M}⊙ ), length (10-276 pc), linear mass density (54-8625 {M}⊙ pc-1), aspect ratio, linearity, velocity gradient, temperature, fragmentation, Galactic location, and orientation angle. The filaments concentrate along major spiral arms. They are widely distributed across the Galactic disk, with 50% located within ±20 pc from the Galactic mid-plane and 27% run in the center of spiral arms. An order of 1% of the molecular ISM is confined in large filaments. Massive star formation is more favorable in large filaments compared to elsewhere. This is the first comprehensive catalog of large filaments that can be useful for a quantitative comparison with spiral structures and numerical simulations.

SEDIGISM: Structure, excitation, and dynamics of the inner Galactic interstellar medium

NASA Astrophysics Data System (ADS)

Schuller, F.; Csengeri, T.; Urquhart, J. S.; Duarte-Cabral, A.; Barnes, P. J.; Giannetti, A.; Hernandez, A. K.; Leurini, S.; Mattern, M.; Medina, S.-N. X.; Agurto, C.; Azagra, F.; Anderson, L. D.; Beltrán, M. T.; Beuther, H.; Bontemps, S.; Bronfman, L.; Dobbs, C. L.; Dumke, M.; Finger, R.; Ginsburg, A.; Gonzalez, E.; Henning, T.; Kauffmann, J.; Mac-Auliffe, F.; Menten, K. M.; Montenegro-Montes, F. M.; Moore, T. J. T.; Muller, E.; Parra, R.; Perez-Beaupuits, J.-P.; Pettitt, A.; Russeil, D.; Sánchez-Monge, Á.; Schilke, P.; Schisano, E.; Suri, S.; Testi, L.; Torstensson, K.; Venegas, P.; Wang, K.; Wienen, M.; Wyrowski, F.; Zavagno, A.

2017-05-01

Context. The origin and life-cycle of molecular clouds are still poorly constrained, despite their importance for understanding the evolution of the interstellar medium. Many large-scale surveys of the Galactic plane have been conducted recently, allowing for rapid progress in this field. Nevertheless, a sub-arcminute resolution global view of the large-scale distribution of molecular gas, from the diffuse medium to dense clouds and clumps, and of their relationshipto the spiral structure, is still missing. Aims: We have carried out a systematic, homogeneous, spectroscopic survey of the inner Galactic plane, in order to complement the many continuum Galactic surveys available with crucial distance and gas-kinematic information. Our aim is to combine this data set with recent infrared to sub-millimetre surveys at similar angular resolutions. Methods: The SEDIGISM survey covers 78 deg2 of the inner Galaxy (-60°≤ℓ≤ 18°, |b|≤ 0.5°) in the J = 2-1 rotational transition of 13CO. This isotopologue of CO is less abundant than 12CO by factors up to 100. Therefore, its emission has low to moderate optical depths, and higher critical density, making it an ideal tracer of the cold, dense interstellar medium. The data have been observed with the SHFI single-pixel instrument at APEX. The observational setup covers the 13CO(2-1) and C18O(2-1) lines, plus several transitions from other molecules. Results: The observations have been completed. Data reduction is in progress, and the final data products will be made available in the near future. Here we give a detailed description of the survey and the dedicated data reduction pipeline. To illustrate the scientific potential of this survey, preliminary results based on a science demonstration field covering -20°≤ℓ ≤ -18.5° are presented. Analysis of the 13CO(2-1) data in this field reveals compact clumps, diffuse clouds, and filamentary structures at a range of heliocentric distances. By combining our data with data in the (1-0) transition of CO isotopologues from the ThrUMMS survey, we are able to compute a 3D realization of the excitation temperature and optical depth in the interstellar medium. Ultimately, this survey will provide a detailed, global view of the inner Galactic interstellar medium at an unprecedented angular resolution of 30''. This publication is based on data acquired with the Atacama Pathfinder EXperiment (APEX) under programmes 092.F-9315(A) and 193.C-0584(A). APEX is a collaboration between the Max-Planck-Institut für Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory.Full Table 5 and Table A.1 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/601/A124

Inverse Interscale Transport of the Reynolds Shear Stress in Plane Couette Turbulence

NASA Astrophysics Data System (ADS)

Kawata, Takuya; Alfredsson, P. Henrik

2018-06-01

Interscale interaction between small-scale structures near the wall and large-scale structures away from the wall plays an increasingly important role with increasing Reynolds number in wall-bounded turbulence. While the top-down influence from the large- to small-scale structures is well known, it has been unclear whether the small scales near the wall also affect the large scales away from the wall. In this Letter we show that the small-scale near-wall structures indeed play a role to maintain the large-scale structures away from the wall, by showing that the Reynolds shear stress is transferred from small to large scales throughout the channel. This is in contrast to the turbulent kinetic energy transport which is from large to small scales. Such an "inverse" interscale transport of the Reynolds shear stress eventually supports the turbulent energy production at large scales.

Motion and properties of nuclear radio components in Seyfert galaxies seen with VLBI

NASA Astrophysics Data System (ADS)

Middelberg, E.; Roy, A. L.; Nagar, N. M.; Krichbaum, T. P.; Norris, R. P.; Wilson, A. S.; Falcke, H.; Colbert, E. J. M.; Witzel, A.; Fricke, K. J.

2004-04-01

We report EVN, MERLIN and VLBA observations at 18 cm, 6 cm and 3.6 cm of the Seyfert galaxies NGC 7674, NGC 5506, NGC 2110 and Mrk 1210 to study their structure and proper motions on pc scales and to add some constraints on the many possible causes of the radio-quietness of Seyferts. The component configurations in NGC 7674 and NGC 2110 are simple, linear structures, whereas the configurations in NGC 5506 and Mrk 1210 have multiple components with no clear axis of symmetry. We suggest that NGC 7674 is a low-luminosity compact symmetric object. Comparing the images at different epochs, we find a proper motion in NGC 7674 of (0.92±0.07) c between the two central components separated by 282 pc and, in NGC 5506, we find a 3 σ upper limit of 0.50 c for the components separated by 3.8 pc. Our results confirm and extend earlier work showing that the outward motion of radio components in Seyfert galaxies is non-relativistic on pc scales. We briefly discuss whether this non-relativistic motion is intrinsic to the jet-formation process or results from deceleration of an initially relativistic jet by interaction with the pc or sub-pc scale interstellar medium. We combined our sample with a list compiled from the literature of VLBI observations made of Seyfert galaxies, and found that most Seyfert nuclei have at least one flat-spectrum component on the VLBI scale, which was not seen in the spectral indices measured at arcsec resolution. We found also that the bimodal alignment of pc and kpc radio structures displayed by radio galaxies and quasars is not displayed by this sample of Seyferts, which shows a uniform distribution of misalignment between 0° and 90°. The frequent misalignment could result from jet precession or from deflection of the jet by interaction with gas in the interstellar medium.

Interstellar Matters: Neutral Hydrogen and the Galactic Magnetic Field

NASA Astrophysics Data System (ADS)

Verschuur, Gerrit; Schmelz, Joan T.; Asgari-Targhi asgari-Targhi, M.

2018-01-01

The physics of the interstellar medium was revolutionized by the observations of the Galactic Arecibo L-Band Feed Array (GALFA) HI survey done at the Arecibo Observatory. The high-resolution, high-sensitivity, high-dynamic- range images show complex, tangled, extended filaments, and reveal that the fabric of the neutral interstellar medium is deeply tied to the structure of the ambient magnetic field. This discovery prompts an obvious question – how exactly is the interstellar {\\it neutral} hydrogen being affected by the galactic magnetic field? We look into this question by examining a set of GALFA-HI data in great detail. We have chosen a long, straight filament in the southern galactic sky. This structure is both close by and isolated in velocity space. Gaussian analysis of profiles both along and across the filament reveal internal structure – braided strands that can be traced through the simplest part, but become tangled in more complex segments. These braids do not resemble in any way the old spherical HI clouds and rudimentary pressure balance models that were used to explain the pre-GALFA- HI interstellar medium. It is clear that these structures are created, constrained, and dominated by magnetic fields. Like many subfields of astronomy before it, e.g., physics of the solar coronal, extragalactic radio jets, and pulsar environment, scientists are confronted with observations that simply cannot be explained by simple hydrodynamics and are forced to consider magneto-hydrodynamics.

IUE observations of neutral hydrogen and deuterium in the local interstellar medium

NASA Technical Reports Server (NTRS)

Landsman, W. B.; Murthy, J.; Henry, R. C.; Moos, H. W.; Linsky, J. L.

1986-01-01

Small-aperture, high-dispersion IUE spectra have been obtained of seven late-type stars that, in general, confirm previous Copernicus results concerning the distribution of hydrogen and deuterium in the local interstellar medium. In addition, the IUE Ly Alpha spectra of Altair, and of the Alpha Cen components, suggest that multiple velocity components exist in these two directions.

Probing the Local Bubble with diffuse interstellar bands. I. Project overview and southern hemisphere survey

NASA Astrophysics Data System (ADS)

Bailey, Mandy; van Loon, Jacco Th.; Farhang, Amin; Javadi, Atefeh; Khosroshahi, Habib G.; Sarre, Peter J.; Smith, Keith T.

2016-01-01

Context. The Sun traverses a low-density, hot entity called the Local Bubble. Despite its relevance to life on Earth, the conditions in the Local Bubble and its exact configuration are not very well known. Besides that, there is some unknown interstellar substance that causes a host of absorption bands across the optical spectrum, called diffuse interstellar bands (DIBs). Aims: We have started a project to chart the Local Bubble in a novel way and learn more about the carriers of the DIBs, by using DIBs as tracers of diffuse gas and environmental conditions. Methods: We conducted a high signal-to-noise spectroscopic survey of 670 nearby early-type stars to map DIB absorption in and around the Local Bubble. The project started with a southern hemisphere survey conducted at the European Southern Observatory's New Technology Telescope and has since been extended to an all-sky survey using the Isaac Newton Telescope. Results: In this first paper in the series, we introduce the overall project and present the results from the southern heiphere survey. We make aviable a catalogue of equivalent-width measurements of the DIBs at 5780, 5797, 5850, 6196, 6203, 6270, 6283, and 6614 Å, of the interstellar Na I D lines at 5890 and 5896 Å, and of the stellar He I line at 5876 Å. We find that the 5780 Å DIB is relatively strong throughout, as compared to the 5797 Å DIB, but especially within the Local Bubble and at the interface iwth a more neutral medium. The 6203 Å DIB shows similar behaviour with respect to the 6196 Å DIB. Some nearby stars show surprisingly strong DIBs, whereas some distant stars show very weak DIBs, indicating small-scale structure within, as well as outside, the Local Bubble. The sight lines with non-detections trace the extent of the Local Bubble especially clearly and show it opening out into the halo. Conclusions: The Local Bubble has a wall that is in contact with hot gas and/or a harsh interstellar radiation field. That wall is perforated, though, causing leakage of radiation and possibly hot gas. On the other hand, compact self-shielded cloudlets are present much closer to the Sun, probably within the Local Bubble itself. As for the carriers of the DIBs, our observations confirm the notion that these are large molecules whose differences in behaviour are mainly governed by their differing resilience and/or electrical charge, with more subtle differences possibly related to varying excitation. Full Tables 1 and 3 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or I http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/585/A12

Carbon atom clusters in random covalent networks: PAHs as an integral component of interstellar HAC

NASA Astrophysics Data System (ADS)

Jones, A. P.

1990-11-01

Using a random covalent network (RCN) model for the structure of hydrogenated amorphorous carbon (HAC) and the available laboratory data, it is shown that aromatic species are a natural consequence of the structure of amorphous carbons formed in the laboratory. Amorphous carbons in the interstellar medium are therefore likely to contain a significant fraction of Polycyclic aromatic hydrocarbons (PAH) species within the 'amorphous' matrix making up these materials. This aromatic component can be produced in situ during the accretion of gas phase carbon species on to grains in the interstellar medium under hydrogen-poor conditions, or subsequent to deposition as a result of photolysis (photodarkening). The fraction of interstellar carbon present in HAC in the form of PAHs, based upon a RCN model, is consistent with the observed Unidentified infrared (UIR) emission features.

Time-dependent MHD modeling of the global structure of the heliosphere

NASA Technical Reports Server (NTRS)

Liewer, P. C.; Brackbill, J. U.; Karmesin, S. Roy

1995-01-01

We present results from time-dependent modeling of the global structure of the heliosphere with neutral and magnetic field effects included. The magnetic field is assumed parallel to the interstellar flow in this two-dimensional axisymmetric model; the neutrals are treated as a fluid. The effects of interstellar neutrals and the interplanetary magnetic field on the location of the termination shock are studied using the most recent estimate of the interstellar medium parameters, results will be compared to those of Baranov and Zaitsev. The effect of the solar wind - VLISM interaction on the density and velocity of interstellar neutrals within the heliosphere will also be presented and related to observations. The response of the termination shock to the solar cycle variation in the solar wind will be compared to the response found previously using an axisymmetric hydrodynamic model without neutrals.

Small scale structure on cosmic strings

NASA Technical Reports Server (NTRS)

Albrecht, Andreas

1989-01-01

The current understanding of cosmic string evolution is discussed, and the focus placed on the question of small scale structure on strings, where most of the disagreements lie. A physical picture designed to put the role of the small scale structure into more intuitive terms is presented. In this picture it can be seen how the small scale structure can feed back in a major way on the overall scaling solution. It is also argued that it is easy for small scale numerical errors to feed back in just such a way. The intuitive discussion presented here may form the basis for an analytic treatment of the small scale structure, which argued in any case would be extremely valuable in filling the gaps in the present understanding of cosmic string evolution.

Properties of nearby interstellar hydrogen deduced from Lyman-alpha sky background measurements

NASA Technical Reports Server (NTRS)

Thomas, G. E.

1972-01-01

For a sufficiently rapid relative motion of the solar system and the nearby interstellar gas, neutral atoms may be expected to penetrate the heliosphere before becoming ionized. Recent satellite measurements of the Lyman alpha emission above the geocorona indicate such an interstellar wind of neutral hydrogen emerging from the direction of Sagittarius and reaching to within a few astronomical units of the sun. A detailed model of the scattering of solar Lyman alpha from the spatial distribution of neutral hydrogen in interplanetary space is presented. This asymmetric distribution is established by solar wind and solar ultraviolet ionization processes along the trajectories of the incoming hydrogen atoms. The values of the interstellar density, the relative velocity, and the gas temperature are adjusted to agree with the Lyman alpha measurements. The results may be interpreted in terms of two models, the cold model and the hot model of the interstellar gas, depending on whether galactic Lyman alpha emission is present at its maximum allowable value or negligibly small.

SILICATE COMPOSITION OF THE INTERSTELLAR MEDIUM

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fogerty, S.; Forrest, W.; Watson, D. M.

2016-10-20

The composition of silicate dust in the diffuse interstellar medium and in protoplanetary disks around young stars informs our understanding of the processing and evolution of the dust grains leading up to planet formation. An analysis of the well-known 9.7 μ m feature indicates that small amorphous silicate grains represent a significant fraction of interstellar dust and are also major components of protoplanetary disks. However, this feature is typically modeled assuming amorphous silicate dust of olivine and pyroxene stoichiometries. Here, we analyze interstellar dust with models of silicate dust that include non-stoichiometric amorphous silicate grains. Modeling the optical depth alongmore » lines of sight toward the extinguished objects Cyg OB2 No. 12 and ζ Ophiuchi, we find evidence for interstellar amorphous silicate dust with stoichiometry intermediate between olivine and pyroxene, which we simply refer to as “polivene.” Finally, we compare these results to models of silicate emission from the Trapezium and protoplanetary disks in Taurus.« less

Efficient radiative transfer methods for continuum and line transfer in large three-dimensional models

NASA Astrophysics Data System (ADS)

Juvela, Mika J.

The relationship between physical conditions of an interstellar cloud and the observed radiation is defined by the radiative transfer problem. Radiative transfer calculations are needed if, e.g., one wants to disentangle abundance variations from excitation effects or wants to model variations of dust properties inside an interstellar cloud. New observational facilities (e.g., ALMA and Herschel) will bring improved accuracy both in terms of intensity and spatial resolution. This will enable detailed studies of the densest sub-structures of interstellar clouds and star forming regions. Such observations must be interpreted with accurate radiative transfer methods and realistic source models. In many cases this will mean modelling in three dimensions. High optical depths and observed wide range of linear scales are, however, challenging for radiative transfer modelling. A large range of linear scales can be accessed only with hierarchical models. Figure 1 shows an example of the use of a hierarchical grid for radiative transfer calculations when the original model cloud (L=10 pc, =500 cm-3) was based a MHD simulation carried out on a regular grid (Juvela & Padoan, 2005). For computed line intensities an accuracy of 10% was still reached when the number of individual cells (and the run time) was reduced by a factor of ten. This illustrates how, as long as cloud is not extremely optically thick, most of the emission comes from a small sub-volume. It is also worth noting that while errors are ~10% for any given point they are much smaller when compared with intensity variations. In particular, calculations on hierarchical grid recovered the spatial power spectrum of line emission with very good accuracy. Monte Carlo codes are used widely in both continuum and line transfer calculations. Like any lambda iteration schemes these suffer from slow convergence when models are optically thick. In line transfer Accelerated Monte Carlo methods (AMC) present a partial solution to this problem (Juvela & Padoan, 2000; Hogerheijde & van der Tak, 2000). AMC methods can be used similarly in continuum calculations to speed up the computation of dust temperatures (Juvela, 2005). The sampling problems associated with high optical depths can be solved with weighted sampling and the handling of models with τV ~ 1000 is perfectly feasible. Transiently heated small dust grains pose another problem because the calculation of their temperature distribution is very time consuming. However, a 3D model will contain thousands of cells at very similar conditions. If dust temperature distributions are calculated only once for such a set an approximate solution can be found in a much shorter time time. (Juvela & Padoan, 2003; see Figure 2a). MHD simulations with Automatic Mesh Refinement (AMR) techniques present an exciting development for the modelling of interstellar clouds. Cloud models consist of a hierarchy of grids with different grid steps and the ratio between the cloud size and the smallest resolution elements can be 106 or even larger. We are currently working on radiative transfer codes (line and continuum) that could be used efficiently on such grids (see Figure 2b). The radiative transfer problem can be solved relatively independently on each of the sub-grids. This means that the use of convergence acceleration methods can be limited to those sub-grids where they are needed and, on the other hand, parallelization of the code is straightforward.

Estimation of a genetically viable population for multigenerational interstellar voyaging: Review and data for project Hyperion

NASA Astrophysics Data System (ADS)

Smith, Cameron M.

2014-04-01

Designing interstellar starships for human migration to exoplanets requires establishing the starship population, which factors into many variables including closed-ecosystem design, architecture, mass and propulsion. I review the central issues of population genetics (effects of mutation, migration, selection and drift) for human populations on such voyages, specifically referencing a roughly 5-generation (c. 150-year) voyage currently in the realm of thought among Icarus Interstellar's Project Hyperion research group. I present several formulae as well as concrete numbers that can be used to help determine populations that could survive such journeys in good health. I find that previously proposed such populations, on the order of a few hundred individuals, are significantly too low to consider based on current understanding of vertebrate (including human) genetics and population dynamics. Population genetics theory, calculations and computer modeling determine that a properly screened and age- and sex-structured total founding population (Nc) of anywhere from roughly 14,000 to 44,000 people would be sufficient to survive such journeys in good health. A safe and well-considered Nc figure is 40,000, an Interstellar Migrant Population (IMP) composed of an Effective Population [Ne] of 23,400 reproductive males and females, the rest being pre- or post-reproductive individuals. This number would maintain good health over five generations despite (a) increased inbreeding resulting from a relatively small human population, (b) depressed genetic diversity due to the founder effect, (c) demographic change through time and (d) expectation of at least one severe population catastrophe over the 5-generation voyage.

Photoionization and heating of a supernova-driven turbulent interstellar medium

NASA Astrophysics Data System (ADS)

Barnes, J. E.; Wood, Kenneth; Hill, Alex S.; Haffner, L. M.

2014-06-01

The diffuse ionized gas (DIG) in galaxies traces photoionization feedback from massive stars. Through three-dimensional photoionization simulations, we study the propagation of ionizing photons, photoionization heating and the resulting distribution of ionized and neutral gas within snapshots of magnetohydrodynamic simulations of a supernova-driven turbulent interstellar medium. We also investigate the impact of non-photoionization heating on observed optical emission line ratios. Inclusion of a heating term which scales less steeply with electron density than photoionization is required to produce diagnostic emission line ratios similar to those observed with the Wisconsin Hα Mapper. Once such heating terms have been included, we are also able to produce temperatures similar to those inferred from observations of the DIG, with temperatures increasing to above 15 000 K at heights |z| ≳ 1 kpc. We find that ionizing photons travel through low-density regions close to the mid-plane of the simulations, while travelling through diffuse low-density regions at large heights. The majority of photons travel small distances (≲100 pc); however some travel kiloparsecs and ionize the DIG.

Dust in the Small Magellanic Cloud

NASA Technical Reports Server (NTRS)

Magalhaes, A. M.

1993-01-01

Observations of reddened stars in the Small Magellanic Cloud (SMC) indicate that the interstellar grains in that galaxy may show distinct optical properties from those in the Galaxy. In a careful study of three SMC objects, Prevot showed that the UV extinction law in the SMC is almost linear with inverse wavelength and the 2200A feature is generally absent. The first results of a program to determine the wavelength dependence of the interstellar optical polarization in the SMC indicate that highly polarized objects are scarce. Our study has uncovered, however, several objects with optical polarization greater than around 1 percent: AZV 126, AZV 211, AZV 221, AZV 398, and AZV 456. The latter two have already had their UV extinction law determined. Our aim was to obtain International Ultraviolet Explorer (IUE) data and determine the UV extinction law also for AZV 126, AZV 211, and AZV 221. AZV 456, which presents a 'galactic' extinction law, has a 'normal' value for its wavelength of maximum polarization, lmax, while AZV 398, which shows a 'typical' SMC extinction curve, shows a somewhat smaller value for such wavelength. AZV 126, AZV 211, and AZV 221 all present extreme small values of lmax but had not had its extinction curve in the W determined yet. We therefore aimed at ultimately determining the extinction law in the direction of these three objects. Such results, in combination with the optical polarization data, have an important bearing on constraining the composition and size distribution of the interstellar dust in the SMC. In the last report, the images gathered with IUE, their processing, and the extinction curves derived from them were described. Such extinction curves and the theoretical models developed to interpret the SMC extinction and polarization data are discussed. Details are presented in an enclosed preprint. The activities in our ongoing polarimetric program of determining the magnetic field structure of the SMC and the images collected at Cerro Tololo Interamerican Observatory during the period are also briefly described. Other activities are also described.

Molecular clouds and the large-scale structure of the galaxy

NASA Technical Reports Server (NTRS)

Thaddeus, Patrick; Stacy, J. Gregory

1990-01-01

The application of molecular radio astronomy to the study of the large-scale structure of the Galaxy is reviewed and the distribution and characteristic properties of the Galactic population of Giant Molecular Clouds (GMCs), derived primarily from analysis of the Columbia CO survey, and their relation to tracers of Population 1 and major spiral features are described. The properties of the local molecular interstellar gas are summarized. The CO observing programs currently underway with the Center for Astrophysics 1.2 m radio telescope are described, with an emphasis on projects relevant to future comparison with high-energy gamma-ray observations. Several areas are discussed in which high-energy gamma-ray observations by the EGRET (Energetic Gamma-Ray Experiment Telescope) experiment aboard the Gamma Ray Observatory will directly complement radio studies of the Milky Way, with the prospect of significant progress on fundamental issues related to the structure and content of the Galaxy.

Paramagnetic alignment of small grains: A novel method for measuring interstellar magnetic fields

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hoang, Thiem; Martin, P. G.; Lazarian, A.

2014-07-20

We present a novel method to measure the strength of interstellar magnetic fields using ultraviolet (UV) polarization of starlight that is in part produced by weakly aligned, small dust grains. We begin with calculating the degrees of the paramagnetic alignment of small (size a ∼ 0.01 μm) and very small (a ∼ 0.001 μm) grains in the interstellar magnetic field due to the Davis-Greenstein relaxation and resonance relaxation. To calculate the degrees of paramagnetic alignment, we use Langevin equations and take into account various interaction processes essential for the rotational dynamics of small grains. We find that the alignment ofmore » small grains is necessary to reproduce the observed polarization in the UV, although the polarization arising from these small grains is negligible at the optical and infrared (IR) wavelengths. Based on fitting theoretical models to observed extinction and polarization curves, we find that the best-fit model for the case with the peak wavelength of polarization λ{sub max} < 0.55 μm requires a higher degree of alignment of small grains than for the typical case with λ{sub max} = 0.55 μm. We interpret the correlation between the systematic increase of the UV polarization relative to maximum polarization (i.e., of p(6 μm{sup –1})/p{sub max}) with λ{sub max}{sup −1} for cases of low λ{sub max} by appealing to the higher degree of alignment of small grains. We utilize the correlation of the paramagnetic alignment of small grains with the magnetic field strength B to suggest a new way to measure B using the observable parameters λ{sub max} and p(6 μm{sup –1})/p{sub max}.« less

An iconic approach to communicating musical concepts in interstellar messages

NASA Astrophysics Data System (ADS)

Vakoch, Douglas A.

2010-12-01

Some characteristics of terrestrial music may be meaningful to extraterrestrial civilizations by virtue of the connection between acoustics and mathematics—both of which might be known by technologically advanced extraterrestrial intelligence. For example, a fundamental characteristic of terrestrial polyphonic music is found the number of tones used various scales, insofar as the number of tones represents a compromise between competing musical demands; the number of tones in a scale, however, also reflects some of the perceptual characteristics of the species developing that music. Thus, in the process of communicating something about the structure of terrestrial music through interstellar messages, additional information about human perceptual and cognitive processes can also be conveyed. This paper also discusses methods for sending signals that bear information through the form of the very frequencies in which the signals are transmitted. If the challenges of creating intelligible messages are greater than often thought, the advantage of reduced conventionality of encoding the message by using an iconic format of this sort may be of significant value. Such an approach would allow the incremental introduction of musical concepts, somewhat akin to the step-by-step tutorials in mathematics and logic that form the basis of Freudenthal's Lincos.

The rate of separation of magnetic lines of force in a random magnetic field.

NASA Technical Reports Server (NTRS)

Jokipii, J. R.

1973-01-01

The mixing of magnetic lines of force, as represented by their rate of separation, as a function of distance along the magnetic field, is considered with emphasis on neighboring lines of force. This effect is particularly important in understanding the transport of charged particles perpendicular to the average magnetic field. The calculation is carried out in the approximation that the separation changes by an amount small compared with the correlation scale normal to the field, in a distance along the field of a few correlation scales. It is found that the rate of separation is very sensitive to the precise form of the power spectrum. Application to the interplanetary and interstellar magnetic fields is discussed, and it is shown that in some cases field lines, much closer together than the correlation scale, separate at a rate which is effectively as rapid as if they were many correlation lengths apart.

A study of the dust distribution and extinction law in Mon R2

NASA Technical Reports Server (NTRS)

Natta, A.; Beckwith, S.; Beck, S. C.; Evans, N. J., II; Moorwood, A. F. M.

1986-01-01

Observations were obtained at wavelengths from 1.5 to 7.5 microns with beams varying in diameter from 4 to 28 arcsec of infrared hydrogen recombination lines toward the Mon R2 IRS1 H II region. It is found that the data cannot be fitted with the extinction law which characterizes the interstellar medium unless the obscuring matter is clumped on a small scale of not greater than 0.3 arcsec; in which case considerable fluctuations in the amount of extinction on scales smaller than 1 arcsec are expected. The data of Simon et al. (1983) suggest a dip in the extinction about 5 arcsec from the 2-micron and radio continuum peak, and rule out models with uniform dust and clump distributions.

Interstellar Probe: The Next Step To Flight

NASA Astrophysics Data System (ADS)

McNutt, Ralph; Zurbuchen, Thomas H.

2016-07-01

In the years following the discovery of the solar wind, the term "heliosphere" was coined and defined as "the region of interplanetary space where the solar wind is flowing supersonically." In June 1971, with the development of the Pioneer probes to Jupiter and beyond well underway, a session of the American Astronautical Society meeting considered scientific exploration reaching beyond the solar system and into the interstellar medium. Despite many discussions, studies, and meetings since, the most recent held under the auspices of the Keck Institute for Space Studies (8-11 September 2014 and 13-15 January 2015), such missions have been relegated to the '"future" due to the large distances and solar system escape speeds contemplated for their execution. In the meantime, the Voyager Interstellar Mission (VIM), consisting of the twin Voyager spacecraft almost 40 years since their respective launches, are making inroads into this region beyond the termination shock of the solar wind, a new region of the solid bodies of the solar system has been opened by the New Horizons flyby of the Pluto system, and the Cassini Ion and Neutral CAmera (INCA) and Interstellar Boundary Explorer (IBEX) have remotely sensed neutral atoms that have provided significant clues to the global structure of the interaction of the solar wind and interstellar medium. It is now time for a dedicated mission to the regime beyond the solar system to explore our galactic environment. A first, near-term implementation can be carried out with the near-current flight system technology. What is also clear is that the high speeds required will limit the spacecraft to a relatively small mass of no more than ~500 kg, regardless of the propulsion details. The recent success of the New Horizons mission at the Pluto system illustrates that with modern technologies, such spacecraft sizes can still accommodate the means to produce paradigm-shifting science, providing for a compelling scientific mission. The Committee on Space Research (COSPAR) has recently established a new Panel on Interstellar Research (PIR) to consider the next steps toward finally making a dedicated Interstellar Probe mission a reality. Crucial tasks are to build consensus amongst the international scientific community for the appropriate scientific campaigns and measurements to be carried out for such a mission, taking into account the new and continuing results from the outer solar system and beyond by VIM, IBEX, New Horizons, and exoplanet observations and studies.

Speckles in interstellar radio-wave scattering

NASA Technical Reports Server (NTRS)

Desai, K. M.; Gwinn, C. R.; Reynolds, J.; King, E. A.; Jauncey, D.; Nicholson, G.; Flanagan, C.; Preston, R. A.; Jones, D. L.

1991-01-01

Observations of speckles in the scattering disk of the Vela pulsar are presented and speckle techniques for studying and circumventing scattering of radio waves by the turbulent interstellar plasma are discussed. The speckle pattern contains, in a hologrammatic fashion, complete information on the structure of the radio source as well as the distribution of the scattering material. Speckle observations of interstellar scattering of radio waves are difficult because of their characteristically short timescales and narrow bandwidths. Here, first observations are presented, taken at 13 cm wavelength with elements of the SHEVE VLBI network, of speckles in interstellar scattering.

First Results from the Interstellar Boundary Explorer (IBEX) Mission

NASA Astrophysics Data System (ADS)

McComas, David J.

2010-03-01

The Interstellar Boundary Explorer (IBEX) is a Small Explorer mission designed to study the global interaction between the heliosphere and the local interstellar medium. IBEX does this by measuring energetic neutral atoms (ENAs) created by both solar wind ions and pickup ions in the solar wind when they charge exchange with cold interstellar neutrals drifting in from the interstellar medium. Because the ENAs are not magnetically confined, some of them propagate back into the inner heliosphere, where IBEX can detect them. IBEX was launched October 19th 2008, using a new launch technique that was also developed as a part of the IBEX project. The first scientific observations from IBEX were of ENAs coming from the Moon-these represented the first ever lunar ENA observations from any spacecraft and provided important information about the universal physical processes of backscatter and neutralization from complex planetary surfaces like the lunar regolith. Since then, IBEX has been collecting its first all-sky maps of heliospheric ENAs and initial direct, in situ observations of interstellar H, He, and O. At the time of this writing, these observations have been submitted and are under review for a special IBEX section of Science magazine nominally scheduled to be published in October 2009.

Global Anisotropies in TeV Cosmic Rays Related to the Sun's Local Galactic Environment from IBEX

NASA Technical Reports Server (NTRS)

Schwadron, N. A.; Adams, F. C.; Christian, E. R.; Desiati, P.; Frisch, P.; Funsten, H. O.; Jokipii, J. R.; McComas, D. J.; Moebius, E.; Zank, G. P.

2014-01-01

Observations with the Interstellar Boundary Explorer (IBEX) have shown enhanced energetic neutral atom (ENA) emission from a narrow, circular ribbon likely centered on the direction of the local interstellar medium (LISM) magnetic field. Here, we show that recent determinations of the local interstellar velocity, based on interstellar atom measurements with IBEX, are consistent with the interstellar modulation of high-energy (tera-electron volts, TeV) cosmic rays and diffusive propagation from supernova sources revealed in global anisotropy maps of ground-based high-energy cosmic-ray observatories (Milagro, Asg, and IceCube). Establishing a consistent local interstellar magnetic field direction using IBEX ENAs at hundreds to thousands of eV and galactic cosmic rays at tens of TeV has wide-ranging implications for the structure of our heliosphere and its interactions with the LISM, which is particularly important at the time when the Voyager spacecraft are leaving our heliosphere.

Global anisotropies in TeV cosmic rays related to the Sun's local galactic environment from IBEX.

PubMed

Schwadron, N A; Adams, F C; Christian, E R; Desiati, P; Frisch, P; Funsten, H O; Jokipii, J R; McComas, D J; Moebius, E; Zank, G P

2014-02-28

Observations with the Interstellar Boundary Explorer (IBEX) have shown enhanced energetic neutral atom (ENA) emission from a narrow, circular ribbon likely centered on the direction of the local interstellar medium (LISM) magnetic field. Here, we show that recent determinations of the local interstellar velocity, based on interstellar atom measurements with IBEX, are consistent with the interstellar modulation of high-energy (tera-electron volts, TeV) cosmic rays and diffusive propagation from supernova sources revealed in global anisotropy maps of ground-based high-energy cosmic-ray observatories (Milagro, Asγ, and IceCube). Establishing a consistent local interstellar magnetic field direction using IBEX ENAs at hundreds to thousands of eV and galactic cosmic rays at tens of TeV has wide-ranging implications for the structure of our heliosphere and its interactions with the LISM, which is particularly important at the time when the Voyager spacecraft are leaving our heliosphere.

J plots: a new method for characterizing structures in the interstellar medium

NASA Astrophysics Data System (ADS)

Jaffa, S. E.; Whitworth, A. P.; Clarke, S. D.; Howard, A. D. P.

2018-06-01

Large-scale surveys have brought about a revolution in astronomy. To analyse the resulting wealth of data, we need automated tools to identify, classify, and quantify the important underlying structures. We present here a method for classifying and quantifying a pixelated structure, based on its principal moments of inertia. The method enables us to automatically detect, and objectively compare, centrally condensed cores, elongated filaments, and hollow rings. We illustrate the method by applying it to (i) observations of surface density from Hi-GAL, and (ii) simulations of filament growth in a turbulent medium. We limit the discussion here to 2D data; in a future paper, we will extend the method to 3D data.

Molecule formation and infrared emission in fast interstellar shocks. I Physical processes

NASA Technical Reports Server (NTRS)

Hollenbach, D.; Mckee, C. F.

1979-01-01

The paper analyzes the structure of fast shocks incident upon interstellar gas of ambient density from 10 to the 7th per cu cm, while focusing on the problems of formation and destruction of molecules and infrared emission in the cooling, neutral post shock gas. It is noted that such fast shocks initially dissociate almost all preexisting molecules. Discussion covers the physical processes which determine the post shock structure between 10 to the 4 and 10 to the 2 K. It is shown that the chemistry of important molecular coolants H2, CO, OH, and H2O, as well as HD and CH, is reduced to a relatively small set of gas phase and grain surface reactions. Also, the chemistry follows the slow conversion of atomic hydrogen into H2, which primarily occurs on grain surfaces. The dependence of this H2 formation rate on grain and gas temperatures is examined and the survival of grains behind fast shocks is discussed. Post shock heating and cooling rates are calculated and an appropriate, analytic, universal cooling function is developed for molecules other than hydrogen which includes opacities from both the dust and the lines.

Stardust Interstellar Preliminary Examination VII: Synchrotron X-Ray Fluorescence Analysis of Six Stardust Interstellar Candidates Measured with the Advanced Photon Source 2-ID-D Microprobe

NASA Technical Reports Server (NTRS)

Allen, Carlton C.; Anderson, David; Bastien, Ron K.; Brenker, Frank E.; Flynn, George J.; Frank, David; Gainsforth, Zack; Sandford, Scott A.; Simionovici, Alexandre S.; Zolensky, Michael E.

2014-01-01

The NASA Stardust spacecraft exposed an aerogel collector to the interstellar dust passing through the solar system. We performed X-ray fluorescence element mapping and abundance measurements, for elements 19 < or = Z < or = 30, on six "interstellar candidates," potential interstellar impacts identified by Stardust@Home and extracted for analyses in picokeystones. One, I1044,3,33, showed no element hot-spots within the designated search area. However, we identified a nearby surface feature, consistent with the impact of a weak, high-speed particle having an approximately chondritic (CI) element abundance pattern, except for factor-of-ten enrichments in K and Zn and an S depletion. This hot-spot, containing approximately 10 fg of Fe, corresponds to an approximately 350 nm chondritic particle, small enough to be missed by Stardust@Home, indicating that other techniques may be necessary to identify all interstellar candidates. Only one interstellar candidate, I1004,1,2, showed a track. The terminal particle has large enrichments in S, Ti, Cr, Mn, Ni, Cu, and Zn relative to Fe-normalized CI values. It has high Al/Fe, but does not match the Ni/Fe range measured for samples of Al-deck material from the Stardust sample return capsule, which was within the field-of-view of the interstellar collector. A third interstellar candidate, I1075,1,25, showed an Al-rich surface feature that has a composition generally consistent with the Al-deck material, suggesting that it is a secondary particle. The other three interstellar candidates, I1001,1,16, I1001,2,17, and I1044,2,32, showed no impact features or tracks, but allowed assessment of submicron contamination in this aerogel, including Fe hot-spots having CI-like Ni/Fe ratios, complicating the search for CI-like interstellar/interplanetary dust.

Stardust Interstellar Preliminary Examination VII: Synchrotron X-ray fluorescence analysis of six Stardust interstellar candidates measured with the Advanced Photon Source 2-ID-D microprobe

NASA Astrophysics Data System (ADS)

Flynn, George J.; Sutton, Steven R.; Lai, Barry; Wirick, Sue; Allen, Carlton; Anderson, David; Ansari, Asna; Bajt, SašA.; Bastien, Ron K.; Bassim, Nabil; Bechtel, Hans A.; Borg, Janet; Brenker, Frank E.; Bridges, John; Brownlee, Donald E.; Burchell, Mark; Burghammer, Manfred; Butterworth, Anna L.; Changela, Hitesh; Cloetens, Peter; Davis, Andrew M.; Doll, Ryan; Floss, Christine; Frank, David; Gainsforth, Zack; Grün, Eberhard; Heck, Philipp R.; Hillier, Jon K.; Hoppe, Peter; Hudson, Bruce; Huth, Joachim; Hvide, Brit; Kearsley, Anton; King, Ashley J.; Leitner, Jan; Lemelle, Laurence; Leroux, Hugues; Leonard, Ariel; Lettieri, Robert; Marchant, William; Nittler, Larry R.; Ogliore, Ryan; Ong, Wei Ja; Postberg, Frank; Price, Mark C.; Sandford, Scott A.; Tresseras, Juan-Angel Sans; Schmitz, Sylvia; Schoonjans, Tom; Silversmit, Geert; Simionovici, Alexandre; Sol, Vicente A.; Srama, Ralf; Stadermann, Frank J.; Stephan, Thomas; Sterken, Veerle; Stodolna, Julien; Stroud, Rhonda M.; Trieloff, Mario; Tsou, Peter; Tsuchiyama, Akira; Tyliszczak, Tolek; Vekemans, Bart; Vincze, Laszlo; von Korff, Joshua; Westphal, Andrew J.; Wordsworth, Naomi; Zevin, Daniel; Zolensky, Michael E.

2014-09-01

The NASA Stardust spacecraft exposed an aerogel collector to the interstellar dust passing through the solar system. We performed X-ray fluorescence element mapping and abundance measurements, for elements 19 ≤ Z ≤ 30, on six "interstellar candidates," potential interstellar impacts identified by Stardust@Home and extracted for analyses in picokeystones. One, I1044,3,33, showed no element hot-spots within the designated search area. However, we identified a nearby surface feature, consistent with the impact of a weak, high-speed particle having an approximately chondritic (CI) element abundance pattern, except for factor-of-ten enrichments in K and Zn and an S depletion. This hot-spot, containing approximately 10 fg of Fe, corresponds to an approximately 350 nm chondritic particle, small enough to be missed by Stardust@Home, indicating that other techniques may be necessary to identify all interstellar candidates. Only one interstellar candidate, I1004,1,2, showed a track. The terminal particle has large enrichments in S, Ti, Cr, Mn, Ni, Cu, and Zn relative to Fe-normalized CI values. It has high Al/Fe, but does not match the Ni/Fe range measured for samples of Al-deck material from the Stardust sample return capsule, which was within the field-of-view of the interstellar collector. A third interstellar candidate, I1075,1,25, showed an Al-rich surface feature that has a composition generally consistent with the Al-deck material, suggesting that it is a secondary particle. The other three interstellar candidates, I1001,1,16, I1001,2,17, and I1044,2,32, showed no impact features or tracks, but allowed assessment of submicron contamination in this aerogel, including Fe hot-spots having CI-like Ni/Fe ratios, complicating the search for CI-like interstellar/interplanetary dust.

Interstellar Probe: First Step to the Stars

NASA Astrophysics Data System (ADS)

McNutt, R. L., Jr.

2017-12-01

The idea of an "Interstellar Probe," a robotic spacecraft traveling into the nearby interstellar medium for the purpose of scientific investigation, dates to the mid-1960s. The Voyager Interstellar Mission (VIM), an "accidental" 40-year-old by-product of the Grand Tour of the solar system, has provided initial answers to the problem of the global heliospheric configuration and the details of its interface with interstellar space. But the twin Voyager spacecraft have, at most, only another decade of lifetime, and only Voyager 1 has emerged from the heliosheath interaction region. To understand the nature of the interaction, a near-term mission to the "near-by" interstellar medium with modern and focused instrumentation remains a compelling priority. Imaging of energetic neutral atoms (ENAs) by the Ion Neutral CAmera (INCA) on Cassini and from the Interstellar Boundary Explorer (IBEX) in Earth orbit have provided significant new insights into the global interaction region but point to discrepancies with our current understanding. Exploring "as far as possible" into "pristine" interstellar space can resolve these. Hence, reaching large heliocentric distances rapidly is a driver for an Interstellar Probe. Such a mission is timely; understanding the interstellar context of exoplanet systems - and perhaps the context for the emergence of life both here and there - hinges upon what we can discover within our own stellar neighborhood. With current spacecraft technology and high-capability launch vehicles, such as the Space Launch System (SLS), a small, but extremely capable spacecraft, could be dispatched to the near-by interstellar medium with at least twice the speed of the Voyagers. Challenges remain with payload mass and power constraints for optimized science measurements. Mission longevity, as experienced by, but not designed into, the Voyagers, communications capability, and radioisotope power system performance and lifetime are solvable engineering challenges. Such a robotic craft can be built, and could be built and launched soon - to enable our first deliberate step to the stars.

Cascades and Dissipative Anomalies in Compressible Fluid Turbulence

NASA Astrophysics Data System (ADS)

Eyink, Gregory L.; Drivas, Theodore D.

2018-02-01

We investigate dissipative anomalies in a turbulent fluid governed by the compressible Navier-Stokes equation. We follow an exact approach pioneered by Onsager, which we explain as a nonperturbative application of the principle of renormalization-group invariance. In the limit of high Reynolds and Péclet numbers, the flow realizations are found to be described as distributional or "coarse-grained" solutions of the compressible Euler equations, with standard conservation laws broken by turbulent anomalies. The anomalous dissipation of kinetic energy is shown to be due not only to local cascade but also to a distinct mechanism called pressure-work defect. Irreversible heating in stationary, planar shocks with an ideal-gas equation of state exemplifies the second mechanism. Entropy conservation anomalies are also found to occur via two mechanisms: an anomalous input of negative entropy (negentropy) by pressure work and a cascade of negentropy to small scales. We derive "4 /5 th-law"-type expressions for the anomalies, which allow us to characterize the singularities (structure-function scaling exponents) required to sustain the cascades. We compare our approach with alternative theories and empirical evidence. It is argued that the "Big Power Law in the Sky" observed in electron density scintillations in the interstellar medium is a manifestation of a forward negentropy cascade or an inverse cascade of usual thermodynamic entropy.

Impact of Distance Determinations on Galactic Structure. I. Young and Intermediate-Age Tracers

NASA Astrophysics Data System (ADS)

Matsunaga, Noriyuki; Bono, Giuseppe; Chen, Xiaodian; de Grijs, Richard; Inno, Laura; Nishiyama, Shogo

2018-06-01

Here we discuss impacts of distance determinations on the Galactic disk traced by relatively young objects. The Galactic disk, ˜40 kpc in diameter, is a cross-road of studies on the methods of measuring distances, interstellar extinction, evolution of galaxies, and other subjects of interest in astronomy. A proper treatment of interstellar extinction is, for example, crucial for estimating distances to stars in the disk outside the small range of the solar neighborhood. We'll review the current status of relevant studies and discuss some new approaches to the extinction law. When the extinction law is reasonably constrained, distance indicators found in today and future surveys are telling us stellar distribution and more throughout the Galactic disk. Among several useful distance indicators, the focus of this review is Cepheids and open clusters (especially contact binaries in clusters). These tracers are particularly useful for addressing the metallicity gradient of the Galactic disk, an important feature for which comparison between observations and theoretical models can reveal the evolution of the disk.

The interstellar medium in the starburst regions of NGC 253 and NGC 3256

NASA Astrophysics Data System (ADS)

Carral, P.; Hollenbach, D. J.; Lord, S. D.; Colgan, S. W. J.; Haas, Michael R.; Rubin, R. H.; Erickson, E. F.

1994-03-01

We discuss observations of the (C II) 158 micrometers, (O I) 63 micrometers, (Si II) 35 micrometers, (O III) 52,88 micrometers, and (S III) 33 micrometers fine-structure transitions toward the central 45 seconds of the starburst galaxies NGC 253 and NGC 3256. The (C II) and (O I) emission probably originates in photodissociated gas at the surfaces of molecular clouds, although a small (less than or approximately 30%) contribution to the (C II) flux from H II regions cannot be ruled out. The (O III) and (S III) lines originate in H II regions and the (Si II) flux is best explained as originating in H II regions with some contribution from photodissociation regions (PDRs). The gas phase silicon abundance is nearly solar in NGC 253, which we interpret as evidence for grain destruction in the starburst region. We find that the photodissociated atomic gas has densities approximately 104/cu cm and temperature 200-300 K. About 2% of the gas is in this phase. The thermal gas pressure in the PDRs, P(PDR)/k approximately 1-3 x 106 K/cu cm, might represent the 'typical' interstellar gas pressure in starburst systems. The Far Ultraviolet (FUV) radiation fields illuminating the clouds are 103-104 stronger than the local Galactic FUV field and come from the contribution of many closely packed O and B stars. For the central 250 pc of NGC 253, we find that the H II gas has an average density ne is approximately 400/cu cm. This corresponds to a thermal pressure P(H II)/k approximately 7 x 106 K/cu cm which is approximately P(PDR)/k, suggesting that the ionized gas is in pressure equilibrium with the photodissociated gas at the surfaces of molecular clouds. The H II gas fills a significant fraction, approximately 0.01-0.3, of the volume between the clouds. The effective temperature of the ionizing stars in NGC 253 is greater than or approximately 34,500 K; 2 x 105 O7.5 stars would produce the observed Lyman continuum photon luminosity. The average separation between the stars is approximately 3 pc. Applying the simple model for the interstellar medium in galactic nuclei of Wolfire, Tielens, & Hollenbach (1990), we find the molecular gas in the central regions of NGC 253 and NGC 3256 to be distributed in a large number (5 x 103 to 5 x 105) of small (0.5-2 pc), dense (approximately 104/cu cm) clouds (or alternatively 'thin-flattened' structures) with volume filling factors 10-3 to 10-2, very different from the local Interstellar Medium (ISM) of the Galaxy. We suggest a self-consistent scenario for the ISM in NGC 253 in which clouds and H II gas are in pressure balance with a supernova-shocked, hot 1-3 x 106 K, low-density (approximately 104/cu cm), all pervasive medium. A feedback mechanism may be indicated in which the pressure generated by the supernovae compresses the molecular clouds and triggers further massive star formation. The similarity of ISM parameters deduced for NGC 253, NGC 3256, and M82 (Lord et al. 1993) suggests that the ISM properties are independent of the luminosity of the starburst or the triggering mechanism, but are rather endemic to starburst systems. The starburst in NGC 3256 appears to be a scaled-up version of the NGC 253 and M82 starbursts.

A modified Rayleigh-Gans-Debye formula for small angle X-ray scattering by interstellar dust grains

NASA Astrophysics Data System (ADS)

Sharma, Subodh K.

2015-05-01

A widely used approximation in studies relating to small angle differential scattering cross-section of X-rays scattered by interstellar dust grains is the well known Rayleigh-Gans-Debye approximation (RGDA). The validity of this approximation, however, is limited only to X-ray energies greater than about 1 keV. At lower energies, this approximation overestimates the exact results. In this paper a modification to the RGDA is suggested. It is shown that a combination of the RGDA with Ramsauer approximation retains the formal simplicity of the RGDA and also yields good agreement with Mie computations at all X-ray energies.

Search for water and life's building blocks in the universe: A summary

NASA Astrophysics Data System (ADS)

Ehrenfreund, Pascale; Kwok, Sun; Bergin, Edwin

2015-08-01

Water and organic compounds are essential ingredients for life on Earth and possibly elsewhere. In gaseous form water acts as a coolant that allows interstellar gas clouds to collapse to form stars, whereas water ice covers small dust particles that agglomerate to form planetesimals and planets. The variety of organic compounds identified in interstellar and circumstellar regions reflects complex reaction schemes in the gaseous and icy/solid state. Interstellar volatiles and refractory materials were processed and radially mixed within the protostellar disk from which our solar system formed. But the dynamic solar nebula was also a source for new materials and the search for water and life’s building blocks on terrestrial planets, most of the outer-solar-system satellites as well as small solar system bodies reveals exciting new findings. The analysis of small bodies and their fragments, meteorites and interplanetary dust particles, sheds lights onto the extraterrestrial delivery process of prebiotic molecules to young planets and the pathways to life’s origin on Earth and possibly elsewhere. We summarize the results of invited and contributed papers of this Focus Meeting which will allow us to better assess the habitability of objects in our solar system and provide constraints for exoplanets.

The Evolution of Dust in the Multiphase Interstellar Medium

NASA Technical Reports Server (NTRS)

Oliversen, Ronald J. (Technical Monitor); Slavin, Jonathan

2003-01-01

Interstellar dust has a profound effect on the structure and evolution of the interstellar medium (ISM) and on the processes by which stars form from it. Dust obscures regions of star formation from view, and the uncertain quantities of elements in dust makes it difficult to measure accurately the abundances of the elements in low density regions. Despite the central importance of dust in astrophysics, we cannot answer some of the most basic questions about it: Why is it that most of the refractory elements are in dust grains? What determines the sizes of interstellar grains? It has been the goal of our proposed theoretical investigations to address these questions by studying the destruction of interstellar grains, and to develop observational diagnostics that can test the models we develop.

Terrestrial record of the solar system's oscillation about the galactic plane

NASA Technical Reports Server (NTRS)

Stothers, R. B.

1985-01-01

A new study is presented of the observational evidence pertaining to the theory which attributes the episodic component of the earth's impact cratering record over the past 600 Myr to gravitational encounters between the solar system and interstellar clouds that cause comets to fall into the solar system and impact the earth. Contrary to a claim by Thaddeus and Chanan (1985), the vertical scale height of the clouds seems to be sufficently small and the sun's vertical trajectory sufficiently large for the modulating effect of the sun's galactovertical motion to be detectable in the terrestrial record of impact cratering with at least a 50 percent a priori probability.

Interstellar Ices and Radiation-induced Oxidations of Alcohols

NASA Astrophysics Data System (ADS)

Hudson, R. L.; Moore, M. H.

2018-04-01

Infrared spectra of ices containing alcohols that are known or potential interstellar molecules are examined before and after irradiation with 1 MeV protons at ∼20 K. The low-temperature oxidation (hydrogen loss) of six alcohols is followed, and conclusions are drawn based on the results. The formation of reaction products is discussed in terms of the literature on the radiation chemistry of alcohols and a systematic variation in their structures. The results from these new laboratory measurements are then applied to a recent study of propargyl alcohol. Connections are drawn between known interstellar molecules, and several new reaction products in interstellar ices are predicted.

Self-organization of cosmic radiation pressure instability. II - One-dimensional simulations

NASA Technical Reports Server (NTRS)

Hogan, Craig J.; Woods, Jorden

1992-01-01

The clustering of statistically uniform discrete absorbing particles moving solely under the influence of radiation pressure from uniformly distributed emitters is studied in a simple one-dimensional model. Radiation pressure tends to amplify statistical clustering in the absorbers; the absorbing material is swept into empty bubbles, the biggest bubbles grow bigger almost as they would in a uniform medium, and the smaller ones get crushed and disappear. Numerical simulations of a one-dimensional system are used to support the conjecture that the system is self-organizing. Simple statistics indicate that a wide range of initial conditions produce structure approaching the same self-similar statistical distribution, whose scaling properties follow those of the attractor solution for an isolated bubble. The importance of the process for large-scale structuring of the interstellar medium is briefly discussed.

Interstellar Polycyclic Aromatic Compounds and Astrophysics

NASA Technical Reports Server (NTRS)

Hodgins, Douglas M.; DeVincenzi, Donald (Technical Monitor)

1999-01-01

Polycyclic aromatic compounds (PACs), a class of organic molecules whose structures are characterized by the presence of two or more fused aromatic rings, have been the subject of astrophysical interest for nearly two decades. Large by interstellar standards (from as few as 20 to perhaps as many as several hundred atoms), it has been suggested that these species are among the most abundant interstellar molecules impacting a wide range of astrophysical phenomena including: the ubiquitous family of infrared emission bands observed in an ever-increasing assortment of astronomical objects; the subtle but rich array of discrete visible/near-infrared interstellar molecular absorption features known as the diffuse interstellar bands (DIBs); the broad near-infrared quasi-continuum observed in a number of nebulae known as excess red emission (ERE); the interstellar ultraviolet extinction curve and broad '2200 Angstrom bump'; the heating/cooling mechanisms of interstellar clouds. Nevertheless, until recently a lack of good-quality laboratory spectroscopic data on PACs under astrophysically relevant conditions (i.e. isolated, ionized molecules; ionized molecular clusters, etc.) has hindered critical evaluation and extension of this model

The Structure of Shocks in the Very Local Interstellar Medium

NASA Astrophysics Data System (ADS)

Mostafavi, P.; Zank, G. P.

2018-02-01

The Voyager 1 magnetometer has detected several shock waves in the very local interstellar medium (VLISM). Interplanetary shock waves can be transmitted across the heliopause (HP) into the VLISM. The first in situ shock observed by Voyager 1 inside the VLISM was remarkably broad and had properties different than those of shocks inside the heliosphere. We present a model of the 2012 VLISM shock, which was observed to be a weak, quasi-perpendicular, low magnetosonic Mach number, low beta, and subcritical shock. Although the heliosphere is a collisionless environment, we show that the VLISM is collisional with respect to the thermal plasma, and that the thermal collisions introduce dissipative terms such as heat conduction and viscosity. The structure of the VLISM shock is determined by thermal proton–proton collisions. VLISM pickup ions (PUIs) do not introduce a significant pressure or dissipation through the shock transition, meaning that the VLISM shock is not mediated by PUIs but only by the thermal gas and magnetic field. Therefore, VLISM shocks are controlled by particle collisions and not by wave–particle interactions. We find that the weak VLISM shock is very broad with a thickness of about 0.12 au, corresponding to the characteristic thermal heat conduction scale length.

Mapping low-frequency carbon radio recombination lines towards Cassiopeia A at 340, 148, 54, and 43 MHz

NASA Astrophysics Data System (ADS)

Salas, P.; Oonk, J. B. R.; van Weeren, R. J.; Wolfire, M. G.; Emig, K. L.; Toribio, M. C.; Röttgering, H. J. A.; Tielens, A. G. G. M.

2018-04-01

Quantitative understanding of the interstellar medium requires knowledge of its physical conditions. Low-frequency carbon radio recombination lines (CRRLs) trace cold interstellar gas and can be used to determine its physical conditions (e.g. electron temperature and density). In this work, we present spatially resolved observations of the low-frequency (≤390 MHz) CRRLs centred around C268α, C357α, C494α, and C539α towards Cassiopeia A on scales of ≤1.2 pc. We compare the spatial distribution of CRRLs with other interstellar medium tracers. This comparison reveals a spatial offset between the peak of the CRRLs and other tracers, which is very characteristic for photodissociation regions and that we take as evidence for CRRLs being preferentially detected from the surfaces of molecular clouds. Using the CRRLs, we constrain the gas electron temperature and density. These constraints on the gas conditions suggest variations of less than a factor of 2 in pressure over ˜1 pc scales, and an average hydrogen density of 200-470 cm-3. From the electron temperature and density maps, we also constrain the ionized carbon emission measure, column density, and path length. Based on these, the hydrogen column density is larger than 1022 cm-2, with a peak of ˜4 × 1022 cm-2 towards the south of Cassiopeia A. Towards the southern peak, the line-of-sight length is ˜40 pc over a ˜2 pc wide structure, which implies that the gas is a thin surface layer on a large (molecular) cloud that is only partially intersected by Cassiopeia A. These observations highlight the utility of CRRLs as tracers of low-density extended H I and CO-dark gas halo's around molecular clouds.

Mapping the Frozen Void

NASA Astrophysics Data System (ADS)

Suutarinen, Aleksi; Fraser, Helen

2013-07-01

Reactions on the surfaces of dust grains play a vital role in the overall chemistry of interstellar matter. These grains become covered by icy layers, which are the largest molecular reservoir in the interstellar medium. Given this, it is surprising that the effect ice has on the overall chain of reactions is poorly characterized. One step on the path of gaining better understanding here is to develop methods of figuring out how much ice is present in these clouds, the links between ice components, and synergy between the ices and gas phase molecules. We do this by examining the absorption spectra of ices on lines of sight towards several stars behind clouds of interstellar matter. From these we can reconstruct spatial maps of the ice distribution on scales of as little as 1000 AU, as a test of the chemical variation within a cloud. By overlapping the ice data with other maps of the same region (gas emission, temperature, density etc) we create combined maps to reveal the astrochemistry of star-forming regions and pre-stellar cores. In this poster we present the continuing results of our ice mapping programme, using data from the AKARI satellite, specifically in slitless spectroscopy observations in the NIR. In this region the key ice features encompass H2O, CO and CO2. The maps illustrate the power of our dedicated AKARI data reduction pipeline, and the novelty of our observing programme. We also detail the next steps' in our ice mapping research. The method is being expanded to include the full 10'x10' AKARI field of view, taking account of image distortion induced by the dispersing optics. These maps are then combined with exiting gas-phase observations and SCUBA maps. The latest attempts at this are shown here. What is clear already is that it is difficult to predict ice abundances from factors such as extinction or gas density alone, and that ice formation and evolution can vary hugely over even very small astronomical scales.

The density structure and star formation rate of non-isothermal polytropic turbulence

NASA Astrophysics Data System (ADS)

Federrath, Christoph; Banerjee, Supratik

2015-04-01

The interstellar medium of galaxies is governed by supersonic turbulence, which likely controls the star formation rate (SFR) and the initial mass function (IMF). Interstellar turbulence is non-universal, with a wide range of Mach numbers, magnetic fields strengths and driving mechanisms. Although some of these parameters were explored, most previous works assumed that the gas is isothermal. However, we know that cold molecular clouds form out of the warm atomic medium, with the gas passing through chemical and thermodynamic phases that are not isothermal. Here we determine the role of temperature variations by modelling non-isothermal turbulence with a polytropic equation of state (EOS), where pressure and temperature are functions of gas density, P˜ ρ ^Γ, T ˜ ρΓ - 1. We use grid resolutions of 20483 cells and compare polytropic exponents Γ = 0.7 (soft EOS), Γ = 1 (isothermal EOS) and Γ = 5/3 (stiff EOS). We find a complex network of non-isothermal filaments with more small-scale fragmentation occurring for Γ < 1, while Γ > 1 smoothes out density contrasts. The density probability distribution function (PDF) is significantly affected by temperature variations, with a power-law tail developing at low densities for Γ > 1. In contrast, the PDF becomes closer to a lognormal distribution for Γ ≲ 1. We derive and test a new density variance-Mach number relation that takes Γ into account. This new relation is relevant for theoretical models of the SFR and IMF, because it determines the dense gas mass fraction of a cloud, from which stars form. We derive the SFR as a function of Γ and find that it decreases by a factor of ˜5 from Γ = 0.7 to 5/3.

Hot Dust in Ultraluminous Infrared Galaxies

NASA Astrophysics Data System (ADS)

Shchekinov, Yu. A.; Vasiliev, E. O.

2017-12-01

Ultraluminous infrared galaxies with total luminosities an order of magnitude greater than that of our galaxy over wavelengths of λλ = 10-800 μm are characterized by a high mass concentration of dust. Because of this, the optical thickness of the interstellar gas is extremely high, especially in the central regions of the galaxies, ranging from 1 at millimeter wavelengths to 104 in the visible. The average temperature of the dust in them is about Td=30 K, but the variations from one galaxy to another are large, with Td=20-70 K. The main source of dust in these galaxies seems to be type II supernova bursts and the main heating source is stars. In addition, given that shock waves from supernovae are an effective mechanism for destruction of interstellar dust in our galaxy and the high optical thickness of the gas with respect to the heating radiation from the stars, this conclusion merits detailed analysis. This paper provides estimates of the dust mass balance and details of its heating in these galaxies based on the example of the ultraluminous galaxy closest to us, Arp 220. It is shown that when supernovae are dominant in the production and destruction of dust in the interstellar gas, the resultant dust mass fraction is close to the observed value for Arp 220. It is also found that the observed stellar population of this galaxy can support a high ( Td ≃ 67 K ) temperature if the dust in its central region is concentrated in small, dense (n 105 cm-3) clouds with radii of 0.003 ≲ pc. Mechanisms capable of maintaining an interstellar gas structure in this state are discussed.

The Enzmann Starship: History and Engineering Appraisal

NASA Astrophysics Data System (ADS)

Crowl, A.; Long, K. F.; Obousy, R.

During his student days Robert Duncan-Enzmann imagined a space vehicle design which he depicted in a watercolour painting and apparently dated 1949. In the 1960s he was heavily involved in space-mission design and introduced the concept of a fusion powered interstellar spacecraft design which utilised a 305 m diameter sphere of frozen Deuterium and a long cylindrical habitat/propulsion section joined onto it by a connecting structural column. The spacecraft was to be manned by a small community of people setting out to colonise nearby stars and the entire vessel would have a launch mass of between 3-12 million tons, most of which would be the propellant. Long time space advocate G. Harry Stine, presented the concept to a wider audience via ``Analog Science Fact & Science Fiction '' magazine in 1973. Stine envisioned the Starship to be part of a wider programme of interstellar exploration, beginning in the 1990s. Although the Enzmann Starship is relatively well known in science fiction circles, it is not well known within the interstellar research community and indeed just as little is known about its creator, Robert Enzmann. Very little has been written about the concept in the academic literature and no modern assessment of its engineering credibility exists. This paper sets out to reliably describe what is known about the Enzmann Starship design and also how the idea originated, based upon what is known to date. In this paper the engineering configuration is described, and a performance assessment is given in the context of modern scientific knowledge. Further information on the history and design of the Enzmann Starship is invited so that this concept can take its rightful place in the history of interstellar spacecraft design proposals.

The Structure and Climate of Size: Small Scale Schooling in an Urban District

ERIC Educational Resources Information Center

LeChasseur, Kimberly

2009-01-01

This study explores mechanisms involved in small scale schooling and student engagement. Specifically, this study questions the validity of arguments for small scale schooling reforms that confound the promised effects of small scale schooling "structures" (such as smaller enrollments, schools-within-schools, and smaller class sizes)…

Measurements of anisotropy in the cosmic microwave background radiation at degree angular scales near the stars Sigma Herculis and Iota Draconis

NASA Technical Reports Server (NTRS)

Clapp, A. C.; Devlin, M. J.; Gundersen, J. O.; Hagmann, C. A.; Hristov, V. V.; Lange, A. E.; Lim, M.; Lubin, P. M.; Mauskopf, P. D.; Meinhold, P. R.

1994-01-01

We present results from two four-frequency observations centered near the stars Sigma Herculis and Iota Draconis during the fourth flight of the Millimeter-wave Anisotropy eXperiment (MAX). The observations were made of 6 deg x 0.6 deg strips of the sky with a 1.4 deg peak to peak sinusoidal chop in all bands. The full width at half maximum (FWHM) beam sizes were calculated 0.55 deg +/- 0.05 deg at 3.5/cm and a 0.75 deg +/- 0.05 deg at 6, 9, and 14/cm. Significant correlated structures were observed at 3.5, 6, and 9/cm. The spectra of these signals are inconsistent with thermal emission from known interstellar dust populations. The extrapolated amplitudes of synchrotron and free-free emission are too small to account for the amplitude of the observed structures. If the observed structures are attributed to cosmic microwave background (CMB) anisotropy with a Gaussian autocorrelation function and a coherence angle of 25 min, then the most probable values at Delta T/T(sub CMB) = 3.1 (sup +1.7 sub -1.3) x 10(exp -5) for the Sigma Herculis scan, and Delta T/T(sub CMB) = 3.3(sup +1.1 sub -1.1) x 10(exp -5) for the Iota Draconis scan (95% confidence upper, lower limits).

Interstellar Initiative Web Page Design

NASA Technical Reports Server (NTRS)

Mehta, Alkesh

1999-01-01

This summer at NASA/MSFC, I have contributed to two projects: Interstellar Initiative Web Page Design and Lenz's Law Relative Motion Demonstration. In the Web Design Project, I worked on an Outline. The Web Design Outline was developed to provide a foundation for a Hierarchy Tree Structure. The Outline would help design a Website information base for future and near-term missions. The Website would give in-depth information on Propulsion Systems and Interstellar Travel. The Lenz's Law Relative Motion Demonstrator is discussed in this volume by Russell Lee.

UV IRRADIATION OF AROMATIC NITROGEN HETEROCYCLES IN INTERSTELLAR ICE ANALOGS

NASA Technical Reports Server (NTRS)

Elsila, J. E.; Bernstein, M. P.; Sanford, S. A.

2005-01-01

Here, we present information on the properties of the ANH quinoline frozen in interstellar water-ice analogs. Quinoline is a two-ring compound structurally analogous to the PAH naphthalene. In this work, binary mixtures of water and quinoline were frozen to create interstellar ice analogs, which were then subjected to ultraviolet photolysis. We will present the infrared spectra of the resulting ices at various temperatures, as well as chromatographic analysis of the residues remaining upon warm-up of these ices to room temperature.

World Ships: The Solar-Photon Sail Option

NASA Astrophysics Data System (ADS)

Matloff, G. L.

The World Ship, a spacecraft large enough to simulate a small-scale terrestrial internal environment, may be the best feasible option to transfer members of a technological civilization between neighboring stars. Because of the projected size of these spacecraft, journey durations of ~1,000 years seem likely. One of the propulsion options for World Ships is the hyper-thin, likely space-manufactured solar-photon sail, unfurled as close to the migrating civilization's home star as possible. Because the sail and associated structure can be wound around the habitat while not in use, it represents the only known ultimately feasible interstellar propulsion system that can be applied for en route galactic-cosmic ray shielding as well as acceleration/ deceleration. This paper reviews the three suggested sail configurations that can be applied to world ship propulsion: parachute, hollow-body and hoop sails. Possible existing and advanced sail and structure materials and the predicted effects on the sail of the near-Sun space environment are reviewed. Consideration of solar-photon-sail World Ships also affects SETI (the Search for Extraterrestrial Intelligence). Can we detect such craft in flight? When in a star's lifetime is migration using such craft likely? What classes of stars are good candidates for solar-sail World-Ship searches?

On the cosmic ray diffusion in a violent interstellar medium

NASA Technical Reports Server (NTRS)

Bykov, A. M.; Toptygin, I. N.

1985-01-01

A variety of the available observational data on the cosmic ray (CR) spectrum, anisotropy and composition are in good agreement with a suggestion on the diffusion propagation of CR with energy below 10(15) eV in the interstellar medium. The magnitude of the CR diffusion coefficient and its energy dependence are determined by interstellar medium (ISM) magnetic field spectra. Direct observational data on magnetic field spectra are still absent. A theoretical model to the turbulence generation in the multiphase ISM is resented. The model is based on the multiple generation of secondary shocks and concomitant large-scale rarefactions due to supernova shock interactions with interstellar clouds. The distribution function for ISM shocks are derived to include supernova statistics, diffuse cloud distribution, and various shock wave propagation regimes. This permits calculation of the ISM magnetic field fluctuation spectrum and CR diffusion coefficient for the hot phase of ISM.

Observations of the impact of starbursts on the interstellar medium in dwarf galaxies

NASA Astrophysics Data System (ADS)

Marlowe, Amanda T.; Heckman, Timothy M.; Wyse, Rosemary F. G.; Schommer, Robert

1995-01-01

Dwarf galaxies play a crucial role in our understanding of the formation and evolution of galaxies, and the concept of supernova-driven mass outflows is a vital ingredient in theories of the structure and evolution of dwarf galaxies. Despite the theoretical importance of these outflows, there is a very limited amount of direct observational evidence for their existence. We have therefore begun a detailed multi-wave-band search for outflows in dwarf (MB greater than or = -18) galaxies with extensive recent or ongoing centrally concentrated star formation. We report the first results of this search in the present paper. Observations of the ionized gas in dwarf amorphous galaxies with centrally concentrated populations of massive stars provide evidence for the large-scale expansion of their expansion of their ionized interstellar media. Fabry-Perot H alpha images reveal the presence of kiloparsec-scale 'superbubbles' and filaments which tend to be oriented along the galaxy minor axis. These structures are comparable in size to the chracteristic optical sizes of the galaxies, and dominate the morphology of the galaxies at low surface brightness in H alpha. Since expanding structure of this size and velocity are not observed in all low-mass galaxies with recent or ongoing star formation, we suggest that we are witnessing transient events that likely have a relatively low 'duty cycle' in such galaxies. That is, we argue that the particular galaxies in the present paper have had significantly elevated star formation rates over the past 107-108 yr (i.e., these are starburst or young poststarburst systems). This interpretation is consistent with the optical colors and emission-line properties of these galaxies.

Observations of the impact of starbursts on the interstellar medium in dwarf galaxies

NASA Technical Reports Server (NTRS)

Marlowe, Amanda T.; Heckman, Timothy M.; Wyse, Rosemary F. G.; Schommer, Robert

1995-01-01

Dwarf galaxies play a crucial role in our understanding of the formation and evolution of galaxies, and the concept of supernova-driven mass outflows is a vital ingredient in theories of the structure and evolution of dwarf galaxies. Despite the theoretical importance of these outflows, there is a very limited amount of direct observational evidence for their existence. We have therefore begun a detailed multi-wave-band search for outflows in dwarf (M(sub B) greater than or = -18) galaxies with extensive recent or ongoing centrally concentrated star formation. We report the first results of this search in the present paper. Observations of the ionized gas in dwarf amorphous galaxies with centrally concentrated populations of massive stars provide evidence for the large-scale expansion of their expansion of their ionized interstellar media. Fabry-Perot H alpha images reveal the presence of kiloparsec-scale 'superbubbles' and filaments which tend to be oriented along the galaxy minor axis. These structures are comparable in size to the chracteristic optical sizes of the galaxies, and dominate the morphology of the galaxies at low surface brightness in H alpha. Since expanding structure of this size and velocity are not observed in all low-mass galaxies with recent or ongoing star formation, we suggest that we are witnessing transient events that likely have a relatively low 'duty cycle' in such galaxies. That is, we argue that the particular galaxies in the present paper have had significantly elevated star formation rates over the past 10(exp 7)-10(exp 8) yr (i.e., these are starburst or young poststarburst systems). This interpretation is consistent with the optical colors and emission-line properties of these galaxies.

Connection of the solar wind with the interstellar medium through numerical modeling

DOE PAGES

Heerikhuisen, J.; Zirnstein, E.; Kawamura, A. D.; ...

2013-06-13

In this article we investigate the interaction between the solar wind (SW) and the local interstellar medium (LISM) using spacecraft data and numerical simulations. In particular, we focus on neutral atom results from NASA's Interstellar Boundary EXplorer (IBEX) mission, and compare these with implementations of our neutral atom models that look at both the energetic neutral atoms (ENAs) which are created as hydrogen of LISM origin interacts with the heliosphere, as well as the transmission of interstellar Oxygen through the heliospheric interface. Lastly, the goal of this work is to better understand the global structure of the heliosphere and itsmore » interaction with the galaxy.« less

Goulds Belt, Interstellar Clouds, and the Eocene Oligocene Helium-3 Enhancement

NASA Technical Reports Server (NTRS)

Rubincam, David Parry

2015-01-01

Drag from hydrogen in the interstellar cloud which formed Gould's Belt may have sent interplanetary dust particle (IDPs) and small meteoroids with embedded helium to the Earth, perhaps explaining part the helium-3 flux increase seen in the sedimentary record near the Eocene-Oligocene transition. Assuming the Solar System passed through part of the cloud, IDPs in the inner Solar System may have been dragged to Earth, while dust and small meteoroids in the asteroid belt up to centimeter size may have been dragged to the resonances, where their orbital eccentricities were pumped up into Earth-crossing orbits; however, this hypotheses does not explain the Popigai and Chesapeake Bay impacts.

Interstellar Interloper 1I/2017 U1: Observations from the NOT and WIYN Telescopes

NASA Astrophysics Data System (ADS)

Jewitt, David; Luu, Jane; Rajagopal, Jayadev; Kotulla, Ralf; Ridgway, Susan; Liu, Wilson; Augusteijn, Thomas

2017-12-01

We present observations of the interstellar interloper 1I/2017 U1 (’Oumuamua) taken during its 2017 October flyby of Earth. The optical colors B - V = 0.70 ± 0.06, V - R = 0.45 ± 0.05, overlap those of the D-type Jovian Trojan asteroids and are incompatible with the ultrared objects that are abundant in the Kuiper Belt. With a mean absolute magnitude H V = 22.95 and assuming a geometric albedo p V = 0.1, we find an average radius of 55 m. No coma is apparent; we deduce a limit to the dust mass production rate of only ˜2 × 10-4 kg s-1, ruling out the existence of exposed ice covering more than a few m2 of the surface. Volatiles in this body, if they exist, must lie beneath an involatile surface mantle ≳0.5 m thick, perhaps a product of prolonged cosmic-ray processing in the interstellar medium. The light curve range is unusually large at ˜2.0 ± 0.2 mag. Interpreted as a rotational light curve the body has axis ratio ≥ {6.3}-1.1+1.3:1 and semi-axes ˜230 m × 35 m. A ≳6:1 axis ratio is extreme relative to most small solar system asteroids and suggests that albedo variations may additionally contribute to the variability. The light curve is consistent with a two-peaked period ˜8.26 hr, but the period is non-unique as a result of aliasing in the data. Except for its unusually elongated shape, 1I/2017 U1 is a physically unremarkable, sub-kilometer, slightly red, rotating object from another planetary system. The steady-state population of similar, ˜100 m scale interstellar objects inside the orbit of Neptune is ˜104, each with a residence time of ˜10 years.

Characterizing gravitational instability in turbulent multicomponent galactic discs

NASA Astrophysics Data System (ADS)

Agertz, Oscar; Romeo, Alessandro B.; Grisdale, Kearn

2015-05-01

Gravitational instabilities play an important role in galaxy evolution and in shaping the interstellar medium (ISM). The ISM is observed to be highly turbulent, meaning that observables like the gas surface density and velocity dispersion depend on the size of the region over which they are measured. In this work, we investigate, using simulations of Milky Way-like disc galaxies with a resolution of ˜ 9 pc, the nature of turbulence in the ISM and how this affects the gravitational stability of galaxies. By accounting for the measured average turbulent scalings of the density and velocity fields in the stability analysis, we can more robustly characterize the average level of stability of the galaxies as a function of scale, and in a straightforward manner identify scales prone to fragmentation. Furthermore, we find that the stability of a disc with feedback-driven turbulence can be well described by a `Toomre-like' Q stability criterion on all scales, whereas the classical Q can formally lose its meaning on small scales if violent disc instabilities occur in models lacking pressure support from stellar feedback.

The Galactic interstellar medium: foregrounds and star formation

NASA Astrophysics Data System (ADS)

Miville-Deschênes, Marc-Antoine

2018-05-01

This review presents briefly two aspects of Galactic interstellar medium science that seem relevant for studying EoR. First, we give some statistical properties of the Galactic foreground emission in the diffuse regions of the sky. The properties of the emission observed in projection on the plane of the sky are then related to how matter is organised along the line of sight. The diffuse atomic gas is multi-phase, with dense filamentary structures occupying only about 1% of the volume but contributing to about 50% of the emission. The second part of the review presents aspect of structure formation in the Galactic interstellar medium that could be relevant for the subgrid physics used to model the formation of the first stars.

GAS PHASE SYNTHESIS OF (ISO)QUINOLINE AND ITS ROLE IN THE FORMATION OF NUCLEOBASES IN THE INTERSTELLAR MEDIUM

DOE Office of Scientific and Technical Information (OSTI.GOV)

Parker, Dorian S. N.; Kaiser, Ralf I.; Kostko, Oleg

Nitrogen-substituted polycyclic aromatic hydrocarbons (NPAHs) have been proposed to play a key role in the astrochemical evolution of the interstellar medium, yet the formation mechanisms of even their simplest prototypes—quinoline and isoquinoline—remain elusive. Here, we reveal a novel concept that under high temperature conditions representing circumstellar envelopes of carbon stars, (iso)quinoline can be synthesized via the reaction of pyridyl radicals with two acetylene molecules. The facile gas phase formation of (iso)quinoline in circumstellar envelopes defines a hitherto elusive reaction class synthesizing aromatic structures with embedded nitrogen atoms that are essential building blocks in contemporary biological-structural motifs. Once ejected from circumstellarmore » shells and incorporated into icy interstellar grains in cold molecular clouds, these NPAHs can be functionalized by photo processing forming nucleobase-type structures as sampled in the Murchison meteorite.« less

Helium glow detector experiment, MA-088. [Apollo Soyuz test project data reduction

NASA Technical Reports Server (NTRS)

Bowyer, C. S.

1978-01-01

Of the two 584 A channels in the helium glow detector, channel #1 appeared to provide data with erratic count rates and undue susceptibility to dayglow and solar contamination possibly because of filter fatigue or failure. Channel #3 data appear normal and of high quality. For this reason only data from this last channel was analyzed and used for detailed comparison with theory. Reduction and fitting techniques are described, as well as applications of the data in the study of nighttime and daytime Hel 584 A emission. A hot model of the interstellar medium is presented. Topics covered in the appendix include: observations of interstellar helium with a gas absorption cell: implications for the structure of the local interstellar medium; EUV dayglow observations with a helium gas absorption cell; and EUV scattering from local interstellar helium at nonzero temperatures: implications for the derivations of interstellar medium parameters.

On the Origin of GEMS

NASA Technical Reports Server (NTRS)

Keller, L. P.; Messenger, S.

2004-01-01

GEMS (glass with embedded metal and sulfides) are a major component of anhydrous interplanetary dust particles (IDPs) their physical and chemical characteristics show marked similarities to contemporary interstellar dust. Recent oxygen isotopic measurements confirm that at least a small fraction (less than 5%) of GEMS are demonstrably presolar, while the remainder have ratios that are indistinguishable from solar values. GEMS with solar oxygen isotopic compositions either (1) had their isotopic compositions homogenized through processing in the interstellar medium (ISM), or (2) formed in the early solar system. Isotopic homogenization necessarily implies chemical homogenization, so (interstellar) GEMS compositions should reflect the average composition of dust in the local ISM. We performed a systematic examination of the bulk chemistry of GEMS in primitive IDPs in order to test this hypothesis.

The Cosmic Century

NASA Astrophysics Data System (ADS)

Longair, Malcolm S.

2013-04-01

Part I. Stars and Stellar Evolution up to the Second World War: 1. The legacy of the nineteenth century; 2. The classification of stellar spectra; 3. Stellar structure and evolution; 4. The end points of stellar evolution; Part II. The Large-Scale Structure of the Universe, 1900-1939: 5. The Galaxy and the nature of spiral nebulae; 6. The origins of astrophysical cosmology; Part III. The Opening up of the Electromagnetic Spectrum: 7. The opening up of the electromagnetic spectrum and the new astronomies; Part IV. The Astrophysics of Stars and Galaxies since 1945: 8. Stars and stellar evolution; 9. The physics of the interstellar medium; 10. The physics of galaxies and clusters of galaxies; 11. High-energy astrophysics; Part V. Astrophysical Cosmology since 1945: 12. Astrophysical cosmology; 13. The determination of cosmological parameters; 14. The evolution of galaxies and active galaxies with cosmic epoch; 15. The origin of galaxies and the large-scale structure of the Universe; 16. The very early Universe; References; Name index; Object index; Subject index.

Deep View of the Large Magellanic Cloud with Six Years of Fermi-LAT Observations

NASA Technical Reports Server (NTRS)

Ackermann, M.; Albert, A.; Atwood, W. B.; Baldini, L.; Ballet, J.; Barbiellini, G.; Bastieri, D.; Bellazzini, R.; Bissaldi, E.; Bloom, E. D.;

Deep view of the Large Magellanic Cloud with six years of Fermi -LAT observations

DOE PAGES

Ackermann, M.

2016-01-27

Context. The nearby Large Magellanic Cloud (LMC) provides a rare opportunity of a spatially resolved view of an external star-forming galaxy in -rays. The LMC was detected at 0.1–100 GeV as an extended source with CGRO/EGRET and using early observations with the Fermi-LAT. The emission was found to correlate with massive star-forming regions and to be particularly bright towards 30 Doradus. Aims. Studies of the origin and transport of cosmic rays (CRs) in the Milky Way are frequently hampered by line-of-sight confusion and poor distance determination. The LMC offers a complementary way to address these questions by revealing whether andmore » how the -ray emission is connected to specific objects, populations of objects, and structures in the galaxy. Methods. We revisited the -ray emission from the LMC using about 73 months of Fermi-LAT P7REP data in the 0.2–100 GeV range. We developed a complete spatial and spectral model of the LMC emission, for which we tested several approaches: a simple geometrical description, template-fitting, and a physically driven model for CR-induced interstellar emission. Results. In addition to identifying PSR J0540-6919 through its pulsations, we find two hard sources positionally coincident with plerion N 157B and supernova remnant N 132D, which were also detected at TeV energies with H.E.S.S. We detect an additional soft source that is currently unidentified. Extended emission dominates the total flux from the LMC. It consists of an extended component of about the size of the galaxy and additional emission from three to four regions with degree-scale sizes. If it is interpreted as CRs interacting with interstellar gas, the large-scale emission implies a large-scale population of ~1–100GeV CRs with a density of ~30% of the local Galactic value. On top of that, the three to four small-scale emission regions would correspond to enhancements of the CR density by factors 2 to 6 or higher, possibly more energetic and younger populations of CRs compared to the large-scale population. An alternative explanation is that this is emission from an unresolved population of at least two dozen objects, such as pulsars and their nebulae or supernova remnants. This small-scale extended emission has a spatial distribution that does not clearly correlate with known components of the LMC, except for a possible relation to cavities and supergiant shells. Conclusions. The Fermi-LAT GeV observations allowed us to detect individual sources in the LMC. Three of the newly discovered sources are associated with rare and extreme objects. The 30 Doradus region is prominent in GeV -rays because PSR J0540-6919 and N 157B are strong emitters. The extended emission from the galaxy has an unexpected spatial distribution, and observations at higher energies and in radio may help to clarify its origin. Key words. Gamma« less

Final Reports of the Stardust ISPE: Seven Probable Interstellar Dust Particles

NASA Technical Reports Server (NTRS)

Allen, Carlton; Sans Tresseras, Juan-Angel; Westphal, Andrew J.; Stroud, Rhonda M.; Bechtel, Hans A.; Brenker, Frank E.; Butterworth, Anna L.; Flynn, George J.; Frank, David R.; Gainsforth, Zack;

The Diffuse Interstellar Bands: an Elderly Astro-Puzzle Rejuvenated

NASA Astrophysics Data System (ADS)

Cox, Nick L. J.

2011-12-01

The interstellar medium constitutes a physically and chemically complex component of galaxies and is important in the cycle of matter and the evolution of stars. From various spectroscopic clues we now know that the interstellar medium is rich in organic compounds. However, identifying the exact nature of all these components remains a challenge. In particular the identification of the so-called diffuse band carriers has been alluding astronomers for almost a century. In recent decades, observational, experimental and theoretical advances have rapidly lead to renewed interest in the diffuse interstellar bands (DIBs). This has been instigated partly by their perceived relation to the infrared aromatic emission bands, the UV extinction bump and far-UV rise, and the growing number of (small) organic molecules identified in space. This chapter gives an overview of the observational properties and behaviour of the DIBs, and their presence throughout the Universe. I will highlight recent progress in identifying their carriers and discuss their potential as tracers and probes of (extra)-Galactic ISM conditions.

TIME VARIATION OF AV AND RV FOR TYPE Ia SUPERNOVAE BEHIND INTERSTELLAR DUST

NASA Astrophysics Data System (ADS)

Huang, Xiaosheng; Biederman, M.; Herger, B.; Aldering, G. S.

2014-01-01

TIME VARIATION OF AV AND RV FOR TYPE Ia SUPERNOVAE BEHIND NON-UNIFORM INTERSTELLAR DUST ABSTRACT We investigate the time variation of the visual extinction, AV, and the total-to-selective extinction ratio, RV, resulting from interstellar dust in front of an expanding photospheric disk of a type Ia supernova (SN Ia). We simulate interstellar dust clouds according to a power law power spectrum and produce extinction maps that either follow a pseudo-Gaussian distribution or a lognormal distribution. The RV maps are produced through a correlation between AV and RV. With maps of AV and RV generated in each case (pseudo-Gaussian and lognormal), we then compute the effective AV and RV for a SN as its photospheric disk expands behind the dust screen. We find for a small percentage of SNe the AV and RV values can vary by a large factor from day to day in the first 40 days after explosion.

Centrosymmetric molecules as possible carriers of diffuse interstellar bands

NASA Astrophysics Data System (ADS)

Kaźmierczak, M.; Schmidt, M. R.; Galazutdinov, G. A.; Musaev, F. A.; Betelesky, Y.; Krełowski, J.

2010-11-01

In this paper, we present new data with interstellar C2 (Phillips bands A 1 Πu-X1 Σ+g), from observations made with the Ultraviolet-Visual Echelle Spectrograph of the European Southern Observatory. We have determined the interstellar column densities and excitation temperatures of C2 for nine Galactic lines. For seven of these, C2 has never been observed before, so in this case the still small sample of interstellar clouds (26 lines of sight), where a detailed analysis of C2 excitation has been made, has increased significantly. This paper is a continuation of previous works where interstellar molecules (C2 and diffuse interstellar bands) have been analysed. Because the sample of interstellar clouds with C2 has increased, we can show that the width and shape of the profiles of some diffuse interstellar bands (6196 and 5797 Å) apparently depend on the gas kinetic and rotational temperatures of C2; the profiles are broader because of the higher values of the gas kinetic and rotational temperatures of C2. There are also diffuse interstellar bands (4964 and 5850 Å) for which this effect does not exist. Based on observations made with ESO telescopes at the Paranal Observatory under programme IDs 266.D-5655(A), 67.C-0281(A), 71.C-0513(C), 67.D-0439(A) and 082.C-0566(A) and at La Silla under programme IDs 078.C-0403(A), 076.C-0164(A) and 073.C-0337(A). Also based on observations made with the 1.8-m telescope in South Korea and the 2-m telescope at the International Centre for Astronomical and Medico-Ecological Research, Terskol, Russia. E-mail: kazmierczak@astri.uni.torun.pl (MK); schmidt@ncac.torun.pl (MRS); runizag@gmail.com (GAG); ybialets@eso.org (YB); jacek@astri.uni.torun.pl (JK)

Double inflation - A possible resolution of the large-scale structure problem

NASA Technical Reports Server (NTRS)

Turner, Michael S.; Villumsen, Jens V.; Vittorio, Nicola; Silk, Joseph; Juszkiewicz, Roman

1987-01-01

A model is presented for the large-scale structure of the universe in which two successive inflationary phases resulted in large small-scale and small large-scale density fluctuations. This bimodal density fluctuation spectrum in an Omega = 1 universe dominated by hot dark matter leads to large-scale structure of the galaxy distribution that is consistent with recent observational results. In particular, large, nearly empty voids and significant large-scale peculiar velocity fields are produced over scales of about 100 Mpc, while the small-scale structure over less than about 10 Mpc resembles that in a low-density universe, as observed. Detailed analytical calculations and numerical simulations are given of the spatial and velocity correlations.

Magnetic seismology of interstellar gas clouds: Unveiling a hidden dimension

NASA Astrophysics Data System (ADS)

Tritsis, Aris; Tassis, Konstantinos

2018-05-01

Stars and planets are formed inside dense interstellar molecular clouds by processes imprinted on the three-dimensional (3D) morphology of the clouds. Determining the 3D structure of interstellar clouds remains challenging because of projection effects and difficulties measuring the extent of the clouds along the line of sight. We report the detection of normal vibrational modes in the isolated interstellar cloud Musca, allowing determination of the 3D physical dimensions of the cloud. We found that Musca is vibrating globally, with the characteristic modes of a sheet viewed edge on, not the characteristics of a filament as previously supposed. We reconstructed the physical properties of Musca through 3D magnetohydrodynamic simulations, reproducing the observed normal modes and confirming a sheetlike morphology.

Modeling the unidentified infrared emission with combinations of polycyclic aromatic hydrocarbons

NASA Technical Reports Server (NTRS)

Allamandola, L. J.; Hudgins, D. M.; Sandford, S. A.

1999-01-01

The infrared emission band spectrum associated with many different interstellar objects can be modeled successfully by using combined laboratory spectra of neutral and positively charged polycyclic aromatic hydrocarbons (PAHs). These model spectra, shown here for the first time, alleviate the principal spectroscopic criticisms previously leveled at the PAH hypothesis and demonstrate that mixtures of free molecular PAHs can indeed account for the overall appearance of the widespread interstellar infrared emission spectrum. Furthermore, these models give us insight into the structures, stabilities, abundances, and ionization balance of the interstellar PAH population. These, in turn, reflect conditions in the emission zones and shed light on the microscopic processes involved in the carbon nucleation, growth, and evolution in circumstellar shells and the interstellar medium.

High energy gamma ray results from the second small astronomy satellite

NASA Technical Reports Server (NTRS)

Fichtel, C. E.; Hartman, R. C.; Kniffen, D. A.; Thompson, D. J.; Bignami, G. F.; Oegelman, H.; Oezel, M. F.; Tuemer, T.

1974-01-01

A high energy (35 MeV) gamma ray telescope employing a thirty-two level magnetic core spark chamber system was flown on SAS 2. The high energy galactic gamma radiation is observed to dominate over the general diffuse radiation along the entire galactic plane, and when examined in detail, the longitudinal and latitudinal distribution seem generally correlated with galactic structural features, particularly with arm segments. The general high energy gamma radiation from the galactic plane, explained on the basis of its angular distribution and magnitude, probably results primarily from cosmic ray interactions with interstellar matter.

Goulds Belt, Interstellar Clouds, and the Eocene-Oligocene Helium-3 Spike

NASA Technical Reports Server (NTRS)

Rubincam, David Parry

2015-01-01

Drag from hydrogen in the interstellar cloud which formed Gould's Belt may have sent small meteoroids with embedded helium to the Earth, perhaps explaining part or all of the (sup 3) He spike seen in the sedimentary record at the Eocene-Oligocene transition. Assuming the Solar System passed through part of the cloud, meteoroids in the asteroid belt up to centimeter size may have been dragged to the resonances, where their orbital eccentricities were pumped up into Earth-crossing orbits.

SYSTEMATIC AND STOCHASTIC VARIATIONS IN PULSAR DISPERSION MEASURES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lam, M. T.; Cordes, J. M.; Chatterjee, S.

2016-04-10

We analyze deterministic and random temporal variations in the dispersion measure (DM) from the full three-dimensional velocities of pulsars with respect to the solar system, combined with electron-density variations over a wide range of length scales. Previous treatments have largely ignored pulsars’ changing distances while favoring interpretations involving changes in sky position from transverse motion. Linear trends in pulsar DMs observed over 5–10 year timescales may signify sizable DM gradients in the interstellar medium (ISM) sampled by the changing direction of the line of sight to the pulsar. We show that motions parallel to the line of sight can alsomore » account for linear trends, for the apparent excess of DM variance over that extrapolated from scintillation measurements, and for the apparent non-Kolmogorov scalings of DM structure functions inferred in some cases. Pulsar motions through atomic gas may produce bow-shock ionized gas that also contributes to DM variations. We discuss the possible causes of periodic or quasi-periodic changes in DM, including seasonal changes in the ionosphere, annual variations of the solar elongation angle, structure in the heliosphere and ISM boundary, and substructure in the ISM. We assess the solar cycle’s role on the amplitude of ionospheric and solar wind variations. Interstellar refraction can produce cyclic timing variations from the error in transforming arrival times to the solar system barycenter. We apply our methods to DM time series and DM gradient measurements in the literature and assess their consistency with a Kolmogorov medium. Finally, we discuss the implications of DM modeling in precision pulsar timing experiments.« less

1I/‘Oumuamua as a Tidal Disruption Fragment from a Binary Star System

NASA Astrophysics Data System (ADS)

Ćuk, Matija

2018-01-01

1I/‘Oumuamua is the first known interstellar small body, probably being only about 100 m in size. Against expectations based on comets, ‘Oumuamua does not show any activity and has a very elongated figure, and it also exhibits undamped rotational tumbling. In contrast, ‘Oumuamua’s trajectory indicates that it was moving with the local stars, as expected from a low-velocity ejection from a relatively nearby system. Here, I assume that ‘Oumuamua is typical of 100 m interstellar objects and speculate on its origins. I find that giant planets are relatively inefficient at ejecting small bodies from inner solar systems of main-sequence stars, and that binary systems offer a much better opportunity for ejections of non-volatile bodies. I also conclude that ‘Oumuamua is not a member of a collisional population, which could explain its dramatic difference from small asteroids. I observe that 100 m small bodies are expected to carry little mass in realistic collisional populations and that occasional events, when whole planets are disrupted in catastrophic encounters, may dominate the interstellar population of 100 m fragments. Unlike the Sun or Jupiter, red dwarf stars are very dense and are capable of thoroughly tidally disrupting terrestrial planets. I conclude that ‘Oumuamua may have originated as a fragment from a planet that was tidally disrupted and then ejected by a dense member of a binary system, which could explain its peculiarities.

The Interstellar Mapping and Acceleration Probe - A Mission to Discover the Origin of Particle Acceleration and its Fundamental Connection to the Global Interstellar Interaction

NASA Astrophysics Data System (ADS)

Schwadron, N.

2017-12-01

Our piece of cosmic real-estate, the heliosphere, is the domain of all human existence - an astrophysical case-history of the successful evolution of life in a habitable system. The Interstellar Boundary Explorer (IBEX) was the first mission to explore the global heliosphere and in concert with Voyager 1 and Voyager 2 is discovering a fundamentally new and uncharted physical domain of the outer heliosphere. In parallel, Cassini/INCA maps the global heliosphere at energies ( 5-55 keV) above those measured by IBEX. The enigmatic IBEX ribbon and the INCA belt were unanticipated discoveries demonstrating that much of what we know or think we understand about the outer heliosphere needs to be revised. The global structure of the heliosphere is highly complex and influenced by competing factors ranging from the local interstellar magnetic field, suprathermal populations both within and beyond the heliopause, and the detailed flow properties of the LISM. Global heliospheric structure and microphysics in turn influences the acceleration of energetic particles and creates feedbacks that modify the interstellar interaction as a whole. The next quantum leap enabled by IMAP will open new windows on the frontier of Heliophysics and probe the acceleration of suprathermal and higher energy particles at a time when the space environment is rapidly evolving. IMAP ultimately connects the acceleration processes observed directly at 1 AU with unprecedented sensitivity and temporal resolution with the global structure of our heliosphere. The remarkable synergy between IMAP, Voyager 1 and Voyager 2 will remain for at least the next decade as Voyager 1 pushes further into the interstellar domain and Voyager 2 moves through the heliosheath. IMAP, like ACE before it, will be a keystone of the Heliophysics System Observatory by providing comprehensive energetic particle, pickup ion, suprathermal ion, neutral atom, solar wind, solar wind heavy ion, and magnetic field observations to diagnose the changing space environment, to discover the fundamental origins of particle acceleration, while discerning the physical processes that control our global heliosphere's interactions with the local interstellar medium.

From Interstellar Polycyclic Aromatic Hydrocarbons and Ice to the Origin of Life

NASA Technical Reports Server (NTRS)

Allamandola, Louis

2004-01-01

Tremendous strides have been made in our understanding of interstellar material over the past twenty years thanks to significant, parallel developments in observational astronomy and laboratory astrophysics. Twenty years ago the composition of interstellar dust was largely guessed at, the concept of ices in dense molecular clouds ignored, and the notion of large, abundant, gas phase, carbon rich molecules widespread throughout the interstellar medium (ISM) considered impossible. Today the composition of dust in the diffuse ISM is reasonably well constrained to cold refractory materials comprised of amorphous and crystalline silicates mixed with an amorphous carbonaceous material containing aromatic structural units and short, branched aliphatic chains. In the dense ISM, the birthplace of stars and planets, these cold dust particles are coated with mixed molecular ices whose composition is very well constrained. Lastly, the signature of carbon-rich polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by early interstellar chemistry standards, is widespread throughout the Universe. The first part of this talk will describe how infrared studies of interstellar space, combined with laboratory simulations, have revealed the composition of interstellar ices (the building blocks of comets) and the high abundance and nature of interstellar PAHs. The laboratory database has now enabled us to gain insight into the identities, abundances, and physical state of many interstellar materials. Within a dense molecular cloud, and especially in the presolar nebula, the materials frozen into the interstellar/precometary ices are photoprocessed by ultraviolet light and produce more complex molecules. The remainder of the presentation will focus on the photochemical evolution of these materials and the possible role of these compounds on the to the carbonaceous components of micrometeorites, they are likely to have been important sources of complex materials on the early Earth and their composition may be related to the origin of life.

The Sensitive Side of Galaxy Formation: How sub-L* Galaxies Accrete, Form Stars, and Enrich the IGM

NASA Astrophysics Data System (ADS)

Oppenheimer, Benjamin

2012-10-01

We propose a series of cosmological zoom simulations specifically targeting the formation and evolution of dwarf and sub-L* galaxies living in halos of 10^11- 10^12 solar masses. The shallow potential wells and low-density environments of these halos provide uniquely sensitive laboratories to understand the physics of galactic feedback, as well as the thermal history of the intergalactic medium, from which these galaxies accrete. Given that 129 orbits of Cycle 18 COS data probing such halos is now being completed, combined with the insufficiency of current cosmological simulations to resolve these halos, the theory is lagging the data. We will remedy this by running zoom simulations of individual halos with 1000-10,000 times greater mass resolution than current cosmological simulations used for similar studies. We aim to resolve the sub-kpc scale of high-velocity cloud-like structures and <100 pc scales of the interstellar medium. We will simulate circumgalactic quasar absorption metal-line and H I statistics using our novel non-equilibrium ionization solver that follows individual ionic states. We will also investigate the delicate balance of accretion, star formation, and feedback required to reproduce the observed stellar properties of these small galaxies. In the spirit of transparency, we will make our simulation results available on a public website to encourage new projects and collaborations with observers and theorists understanding the physics regulating galaxy growth.

An application of an optimal statistic for characterizing relative orientations

NASA Astrophysics Data System (ADS)

Jow, Dylan L.; Hill, Ryley; Scott, Douglas; Soler, J. D.; Martin, P. G.; Devlin, M. J.; Fissel, L. M.; Poidevin, F.

2018-02-01

We present the projected Rayleigh statistic (PRS), a modification of the classic Rayleigh statistic, as a test for non-uniform relative orientation between two pseudo-vector fields. In the application here, this gives an effective way of investigating whether polarization pseudo-vectors (spin-2 quantities) are preferentially parallel or perpendicular to filaments in the interstellar medium. For example, there are other potential applications in astrophysics, e.g. when comparing small-scale orientations with larger scale shear patterns. We compare the efficiency of the PRS against histogram binning methods that have previously been used for characterizing the relative orientations of gas column density structures with the magnetic field projected on the plane of the sky. We examine data for the Vela C molecular cloud, where the column density is inferred from Herschel submillimetre observations, and the magnetic field from observations by the Balloon-borne Large-Aperture Submillimetre Telescope in the 250-, 350- and 500-μm wavelength bands. We find that the PRS has greater statistical power than approaches that bin the relative orientation angles, as it makes more efficient use of the information contained in the data. In particular, the use of the PRS to test for preferential alignment results in a higher statistical significance, in each of the four Vela C regions, with the greatest increase being by a factor 1.3 in the South-Nest region in the 250 - μ m band.

The Zone of Avoidance as an X-ray absorber - the role of the galactic foreground modelling Swift XRT spectra

NASA Astrophysics Data System (ADS)

Racz, I. I.; Bagoly, Z.; Tóth, L. V.; Balázs, L. G.; Horvath, I.; Zahorecz, S.

2018-05-01

Gamma-ray bursts (GRBs) are the most powerful explosive events in the Universe. The prompt gamma emission is followed by an X-ray afterglow that is also detected for over nine hundred GRBs by the Swift BAT and XRT detectors. The X-ray afterglow spectrum bears essential information about the burst, and the surrounding interstellar medium (ISM). Since the radiation travels through the line of sight intergalactic medium and the ISM in the Milky Way, the observed emission is influenced by extragalactic and galactic components. The column density of the Galactic foreground ranges several orders of magnitudes, due to both the large scale distribution of ISM and its small scale structures. We examined the effect of local HI column density on the penetrating X-ray emission, as the first step towards a precise modeling of the measured X-ray spectra. We fitted the X-ray spectra using the Xspec software, and checked how the shape of the initially power low spectrum changes with varying input Galactic HI column density. The total absorbing HI column is a sum of the intrinsic and Galactic component. We also investigated the model results for the intrinsic component varying the Galactic foreground. We found that such variations may alter the intrinsic hydrogen column density up to twenty-five percent. We will briefly discuss its consequences.

Cosmic Star Formation: A Simple Model of the SFRD(z)

NASA Astrophysics Data System (ADS)

Chiosi, Cesare; Sciarratta, Mauro; D’Onofrio, Mauro; Chiosi, Emanuela; Brotto, Francesca; De Michele, Rosaria; Politino, Valeria

2017-12-01

We investigate the evolution of the cosmic star formation rate density (SFRD) from redshift z = 20 to z = 0 and compare it with the observational one by Madau and Dickinson derived from recent compilations of ultraviolet (UV) and infrared (IR) data. The theoretical SFRD(z) and its evolution are obtained using a simple model that folds together the star formation histories of prototype galaxies that are designed to represent real objects of different morphological type along the Hubble sequence and the hierarchical growing of structures under the action of gravity from small perturbations to large-scale objects in Λ-CDM cosmogony, i.e., the number density of dark matter halos N(M,z). Although the overall model is very simple and easy to set up, it provides results that mimic results obtained from highly complex large-scale N-body simulations well. The simplicity of our approach allows us to test different assumptions for the star formation law in galaxies, the effects of energy feedback from stars to interstellar gas, the efficiency of galactic winds, and also the effect of N(M,z). The result of our analysis is that in the framework of the hierarchical assembly of galaxies, the so-called time-delayed star formation under plain assumptions mainly for the energy feedback and galactic winds can reproduce the observational SFRD(z).

Formation and Destruction Processes of Interstellar Dust: From Organic Molecules to carbonaceous Grains

NASA Technical Reports Server (NTRS)

Salama, F.; Biennier, L.

2004-01-01

The study of the formation and destruction processes of cosmic dust is essential to understand and to quantify the budget of extraterrestrial organic molecules. interstellar dust presents a continuous size distribution from large molecules, radicals and ions to nanometer-sized particles to micron-sized grains. The lower end of the dust size distribution is thought to be responsible for the ubiquitous spectral features that are seen in emission in the IR (UIBs) and in absorption in the visible (DIBs). The higher end of the dust-size distribution is thought to be responsible for the continuum emission plateau that is seen in the IR and for the strong absorption seen in the interstellar UV extinction curve. All these spectral signatures are characteristic of cosmic organic materials that are ubiquitous and present in various forms from gas-phase molecules to solid-state grains. Although dust with all its components plays an important role in the evolution of interstellar chemistry and in the formation of organic molecules, little is known on the formation and destruction processes of dust. Recent space observations in the UV (HST) and in the IR (ISO) help place size constraints on the molecular component of carbonaceous IS dust and indicate that small (ie., subnanometer) PAHs cannot contribute significantly to the IS features in the UV and in the IR. Studies of large molecular and nano-sized IS dust analogs formed from PAH precursors have been performed in our laboratory under conditions that simulate diffuse ISM environments (the particles are cold -100 K vibrational energy, isolated in the gas phase and exposed to a high-energy discharge environment in a cold plasma). The species (molecules, molecular fragments, ions, nanoparticles, etc) formed in the pulsed discharge nozzle (PDN) plasma source are detected with a high-sensitivity cavity ring-down spectrometer (CRDS). We will present new experimental results that indicate that nanoparticles are generated in the plasma. From these unique measurements, we derive information on the nature, the size and the structure of interstellar dust particles, the growth and the destruction processes of IS dust and the resulting budget of extraterrestrial organic molecules.

Ion-Neutral Coupling in Solar Prominence

NASA Technical Reports Server (NTRS)

Gilbert, H.; DeVore, C. R.; Karpen, J.; Kucera, T.; Antiochos, S.; Kawashima, R.

2011-01-01

Coupling between ions and neutrals in magnetized plasmas is fundamentally important to many aspects of heliophysics, including our ionosphere, the solar chromosphere, the solar wind interaction with planetary atmospheres, and the interface between the heliosphere and the interstellar medium. Ion-neutral coupling also plays a major role in the physics of solar prominences. By combining theory, modeling, and observations we are working toward a better understanding of the structure and dynamics of partially ionized prominence plasma. Two key questions are addressed in the present work: 1) what physical mechanism(s) sets the cross-field scale of prominence threads? 2) Are ion-neutral interactions responsible for the vertical flows and structure in prominences? We present initial results from a study investigating what role ion-neutral interactions play in prominence dynamics and structure. This research was supported by NASA.

Ionization of Local Interstellar Gas Based on STIS and FUSE spectra of Nearby Stars

NASA Astrophysics Data System (ADS)

Redfield, Seth; Linsky, J. L.

2009-01-01

The ultraviolet contains many resonance line transitions that are sensitive to a range of ionization stages of ions present in the local interstellar medium (LISM). We couple observations of high resolution ultraviolet spectrographs, STIS and GHRS on the Hubble Space Telescope (HST) and the Far-Ultraviolet Spectroscopic Explorer (FUSE) in order to make a comprehensive survey of the ionization structure of the local interstellar medium. In particular, we focus on the sight line toward G191-B2B, a nearby (69 pc) white dwarf. We present interstellar detections of highly ionized elements (e.g., SiIII, CIII, CIV, etc) and compare them directly to neutral or singly ionized LISM detections (e.g., SiII, CII, etc). The extensive observations of G191-B2B provides an opportunity for a broad study of ionization stages of several elements, while a survey of several sight lines provides a comprehensive look at the ionization structure of the LISM. We acknowledge support for this project through NASA FUSE Grant NNX06AD33G.

"CHON" particles: The interstellar component of cometary dust

NASA Technical Reports Server (NTRS)

Lien, David J.

1998-01-01

Interstellar dust is characterized by strong absorption in the ultraviolet and the mid-IR. Current models of interstellar dust are based on three chemically distinct components: a form of carbon (usually graphite), a silicate, and a blend of polycyclic aromatic hydrocarbons or other carbonaceous material. Previous work using effective medium theories to understand the optical properties of cometary dust suggested that an amalgam of materials could reproduce the observed interstellar and cometary dust features. Recently, Lawler and Brownlee (1992) re-analyzed the PIA and PUMA-1 data sets from the Giotto flyby of P/Halley and discovered that the so-called "CHON" particles were actually composed of a blend of carbon-bearing and silicon-bearing materials. Based on effective medium theories, the absorption spectrum of such a material would display the spectral features of each of the components - strong UV absorption from the carbonaceous component and strong absorption in the IR from the silicate component. To test this idea, vapor-deposited samples were created using two different deposition techniques: sputtering with an argon RF magnetron and deposition from an argon plasma torch. Two different compositions were tested: a blend of graphite and silica in a 7:1 ratio and an amalgam of materials whose approximate composition matches the "CHON"-silicate abundances for the uncompressed PIA data set of Lawler and Brownlee: graphite, iron oxide, magnesium oxide, ammonium sulfate, calcium carbonate, and silica in mass ratios of 6:4.3:4:2.2:1:9. The samples were finely ground and pressed into 2" diameter disks using a 40 ton press. In all, four different experiments were performed: one with each of the compositions (C:SiO and "CHON") in both the RF magnetron and the plasma torch chambers. The RF magnetron created a uniform dark thin film on the substrate surface, and the plasma torch created a coating of small (<100 micron) diameter grey particles. The spectra of all four samples show a strong, broad absorption feature at around 220 nm as well as a strong but narrower absorption peak near 10 microns. The RF magnetron sputtered samples showed some sub-structure in the UV, and the peak of the absorption was shifted toward longer wavelengths. The UV absorption in the plasma torch deposited samples have no sub-structure, and the peak absorption is very near 220 nm. Strong absorption near 9 microns is seen in the spectra from both sample preparation techniques, and is consistent with the IR spectra of some terrestrial silicates. Other features, particularly at 6.2 and 8.6 microns, are seen in the interstellar medium. A strong feature near 2 microns is due to absorbed water in the sample. Based on the results of these experiments, there is evidence that a material with a composition similar to that detected in "CHON" particles in the coma of P/Halley have a spectral signature which reproduces the main absorption features of interstellar dust. This suggests that the "CHON" particles could be the interstellar component of cometary dust.

Investigation of low-latitude hydrogen emission in terms of a two-component interstellar gas model

NASA Technical Reports Server (NTRS)

Baker, P. L.; Burton, W. B.

1975-01-01

High-resolution 21-cm hydrogen line observations at low galactic latitude are analyzed to determine the large-scale distribution of galactic hydrogen. Distribution parameters are found by model fitting, optical depth effects are computed using a two-component gas model suggested by the observations, and calculations are made for a one-component uniform spin-temperature gas model to show the systematic departures between this model and data obtained by incorrect treatment of the optical depth effects. Synthetic 21-cm line profiles are computed from the two-component model, and the large-scale trends of the observed emission profiles are reproduced together with the magnitude of the small-scale emission irregularities. Values are determined for the thickness of the galactic hydrogen disk between half density points, the total observed neutral hydrogen mass of the galaxy, and the central number density of the intercloud hydrogen atoms. It is shown that typical hydrogen clouds must be between 1 and 13 pc in diameter and that optical thinness exists on large-scale despite the presence of optically thin gas.

COMPUTING THE DUST DISTRIBUTION IN THE BOW SHOCK OF A FAST-MOVING, EVOLVED STAR

DOE Office of Scientific and Technical Information (OSTI.GOV)

Van Marle, A. J.; Meliani, Z.; Keppens, R.

2011-06-20

We study the hydrodynamical behavior occurring in the turbulent interaction zone of a fast-moving red supergiant star, where the circumstellar and interstellar material collide. In this wind-interstellar-medium collision, the familiar bow shock, contact discontinuity, and wind termination shock morphology form, with localized instability development. Our model includes a detailed treatment of dust grains in the stellar wind and takes into account the drag forces between dust and gas. The dust is treated as pressureless gas components binned per grain size, for which we use 10 representative grain size bins. Our simulations allow us to deduce how dust grains of varyingmore » sizes become distributed throughout the circumstellar medium. We show that smaller dust grains (radius <0.045 {mu}m) tend to be strongly bound to the gas and therefore follow the gas density distribution closely, with intricate fine structure due to essentially hydrodynamical instabilities at the wind-related contact discontinuity. Larger grains which are more resistant to drag forces are shown to have their own unique dust distribution, with progressive deviations from the gas morphology. Specifically, small dust grains stay entirely within the zone bound by shocked wind material. The large grains are capable of leaving the shocked wind layer and can penetrate into the shocked or even unshocked interstellar medium. Depending on how the number of dust grains varies with grain size, this should leave a clear imprint in infrared observations of bow shocks of red supergiants and other evolved stars.« less

Composition, structure, and chemistry of interstellar dust

NASA Technical Reports Server (NTRS)

Tielens, A. G. G. M.; Allamandola, L. J.

1987-01-01

Different dust components present in the interstellar medium (IM) such as amorphous carbon, polycyclic aromatic hydrocarbons, and those IM components which are organic refractory grains and icy grain mantles are discussed as well as their relative importance. The physical properties of grain surface chemistry are discussed with attention given to the surface structure of materials, the adsorption energy and residence time of species on a grain surface, and the sticking probability. Consideration is also given to the contribution of grains to the gas-phase composition of molecular clouds.

Large and small-scale structures in Saturn's rings

NASA Astrophysics Data System (ADS)

Albers, N.; Rehnberg, M. E.; Brown, Z. L.; Sremcevic, M.; Esposito, L. W.

2017-09-01

Observations made by the Cassini spacecraft have revealed both large and small scale structures in Saturn's rings in unprecedented detail. Analysis of high-resolution measurements by the Cassini Ultraviolet Spectrograph (UVIS) High Speed Photometer (HSP) and the Imaging Science Subsystem (ISS) show an abundance of intrinsic small-scale structures (or clumping) seen across the entire ring system. These include self-gravity wakes (50-100m), sub-km structure at the A and B ring edges, and "straw"/"ropy" structures (1-3km).

The Interstellar Medium

NASA Technical Reports Server (NTRS)

Tielens, Alexander G. G. M.

1995-01-01

The Interstellar Medium (ISM) forms an integral part of the lifecycle of stars and the galaxy. Stars are formed by gravitational contraction of interstellar clouds. Over their life, stars return much of their mass to the ISM through winds and supernova explosions, resulting in a slow enrichment in heavy elements. Understanding the origin and evolution of the ISM is a key problem within astrophysics. The KAO has made many important contributions to studies of the interstellar medium both on the macro and on the micro scale. In this overview, I will concentrate on two breakthroughs in the last decade in which KAO observations have played a major role: (1) the importance of large Polycyclic Aromatic Hydrocarbon (PAH) molecules for the ISM (section 3) and (2) the study of Photodissociation Regions (PDRs) as an analog for the diffuse ISM at large (section 4). Appropriately, the micro and macro problem are intricately interwoven in these problems. Finally, section 5 reviews the origin of the (CII) emission observed by COBE.

Complex Organics from Laboratory Simulated Interstellar Ices

NASA Technical Reports Server (NTRS)

Dworkin, J. P.

2003-01-01

Many of the volatiles in interstellar dense clouds exist in ices surrounding dust grains. The low temperatures of these ices (T < 50 K) preclude most chemical reactions, but photolysis can drive reactions that produce a suite of new species, many of which are complex organics. We study the UV and proton radiation processing of interstellar ice analogs to explore links between interstellar chemistry, the organics in comets and meteorites, and the origin of life on Earth. The high D/H ratios in some interstellar species, and the knowledge that many of the organics in primitive meteorites are D-enriched, suggest that such links are plausible. Once identified, these species may serve as markers of interstellar heritage of cometary dust and meteorites. Of particular interest are our findings that UV photolysis of interstellar ice analogs produce molecules of importance in current living organisms, including quinones, amphiphiles, and amino acids. Quinones are essential in vital metabolic roles such as electron transport. Studies show that quinones should be made wherever polycyclic aromatic hydrocarbons are photolyzed in interstellar ices. In the case of anthracene-containing ices, we have observed the production of 9-anthrone and 9,10 anthraquinone, both of which have been observed in the Murchison meteorite. Amphiphiles are also made when mixed molecular ices are photolyzed. These amphiphiles self-assemble into fluorescent vesicles when placed in liquid water, as do Murchison extracts. Both have the ability to trap an ionic dye. Photolysis of plausible ices can also produce alanine, serine, and glycine as well as a number of small alcohols and amines. Flash heating of the room temperature residue generated by such experiments generates mass spectral distributions similar to those of IDPs. The detection of high D/H ratios in some interstellar molecular species, and the knowledge that many of the organics, such as hydroxy and amino acids, in primitive meteorites are D-enriched provides evidence for a connection between intact organic material in the interstellar medium and in meteorites. Thus, some of the oxidized aromatics, amphiphiles, amino acids, hydroxy acids, and other compounds found in meteorites may have had an interstellar ancestry and not solely a product of parent body aqueous alteration. Such compounds should also be targeted for searches of organics in cometary dust.

Interstellar grains within interstellar grains

NASA Technical Reports Server (NTRS)

Bernatowicz, Thomas J.; Amari, Sachiko; Zinner, Ernst K.; Lewis, Roy S.

1991-01-01

Five interstellar graphite spherules extracted from the Murchison carbonaceous meteorite are studied. The isotopic and elemental compositions of individual particles are investigated with the help of an ion microprobe, and this analysis is augmented with structural studies of ultrathin sections of the grain interiors by transmission electron microscopy. As a result, the following procedure for the formation of the interstellar graphite spherule bearing TiC crystals is inferred: (1) high-temperature nucleation and rapid growth of the graphitic carbon spherule in the atmosphere of a carbon-rich star, (2) nucleation and growth of TiC crystals during continued growth of the graphitic spherule and the accretion of TiC onto the spherule, (3) quenching of the graphite growth process by depletion of C or by isolation of the spherule before other grain types could condense.

Interstellar Dust in the Heliosheath: Tentative Discovery of the Magnetic Wall of the Heliosphere

NASA Astrophysics Data System (ADS)

Frisch, P. C.

2005-12-01

The evident identification of interstellar dust grains entrained in the magnetic wall of the heliosphere is reported. It is shown that the distribution of dust grains causing the weak polarization of light from nearby stars is consistent with polarization by small charged interstellar dust grains captured in the heliosphere magnetic wall (Tinbergen 1982, Frisch 2005). There is an offset between the deflected small charged polarizing dust grains, radius less than 0.2 microns, and the undeflected large grain population, radius larger than 0.2 microns. The region of maximum polarization is towards ecliptic coordinates lambda,beta = 295,0 deg, which is offset along the ecliptic longitude by about 35 deg from the heliosphere nose and extends to low ecliptic latitudes where the heliosphere magnetic wall is expected. An offset is also found between the best aligned dust grains, near lambda=281 deg to 220 deg, and the upwind direction of the undeflected inflow of large grains seen by Ulysses and Galileo. In the aligned-grain region, the polarization strength anti-correlates with ecliptic latitude, indicating that the magnetic wall was predominantly at negative ecliptic latitudes when these data were acquired. These data are consistent with model predictions for an interstellar magnetic field which is tilted by 60 deg with respect to the ecliptic plane, and parallel to the galactic plane. References: Tinbergen, 1982: AA, v105, p53; Frisch, 2005: to appear in ApJL.

Detection of organic matter in interstellar grains.

PubMed

Pendleton, Y J

1997-06-01

Star formation and the subsequent evolution of planetary systems occurs in dense molecular clouds, which are comprised, in part, of interstellar dust grains gathered from the diffuse interstellar medium (DISM). Radio observations of the interstellar medium reveal the presence of organic molecules in the gas phase and infrared observational studies provide details concerning the solid-state features in dust grains. In particular, a series of absorption bands have been observed near 3.4 microns (approximately 2940 cm-1) towards bright infrared objects which are seen through large column densities of interstellar dust. Comparisons of organic residues, produced under a variety of laboratory conditions, to the diffuse interstellar medium observations have shown that aliphatic hydrocarbon grains are responsible for the spectral absorption features observed near 3.4 microns (approximately 2940 cm-1). These hydrocarbons appear to carry the -CH2- and -CH3 functional groups in the abundance ratio CH2/CH3 approximately 2.5, and the amount of carbon tied up in this component is greater than 4% of the cosmic carbon available. On a galactic scale, the strength of the 3.4 microns band does not scale linearly with visual extinction, but instead increases more rapidly for objects near the Galactic Center. A similar trend is noted in the strength of the Si-O absorption band near 9.7 microns. The similar behavior of the C-H and Si-O stretching bands suggests that these two components may be coupled, perhaps in the form of grains with silicate cores and refractory organic mantles. The ubiquity of the hydrocarbon features seen in the near infrared near 3.4 microns throughout out Galaxy and in other galaxies demonstrates the widespread availability of such material for incorporation into the many newly forming planetary systems. The similarity of the 3.4 microns features in any organic material with aliphatic hydrocarbons underscores the need for complete astronomical observational coverage in the 2-30 microns region, of lines of sight which sample dust in both dense and diffuse interstellar clouds, in order to uniquely specify the composition of interstellar organics. This paper reviews the information available from ground-based observations, although currently the Infrared Satellite Observatory is adding to our body of knowledge on this subject by providing more extensive wavelength coverage. The Murchison carbonaceous meteorite has also been used as an analog to the interstellar observations and has revealed a striking similarity between the light hydrocarbons in the meteorite and the ISM; therefore this review includes comparisons with the meteoritic analog as well as with relevant laboratory residues. Fundamental to the evolution of the biogenic molecules, to the process of planetary system formation, and perhaps to the origin of life, is the connection between the organic material found in the interstellar medium and that incorporated in the most primitive solar system bodies.

Solar lens mission concept for interstellar exploration

NASA Astrophysics Data System (ADS)

Brashears, Travis; Lubin, Philip; Turyshev, Slava; Shao, Michael; Zhang, Qicheng

2015-09-01

The long standing approach to space travel has been to incorporate massive on-board electronics, probes and propellants to achieve space exploration. This approach has led to many great achievements in science, but will never help to explore the interstellar medium. Fortunately, a paradigm shift is upon us in how a spacecraft is constructed and propelled. This paper describes a mission concept to get to our Sun's Gravity Lens at 550AU in less than 10 years. It will be done by using DE-STAR, a scalable solar-powered phased-array laser in Earth Orbit, as a directed energy photon drive of low-mass wafersats. [1] [2] [3] [4] [5] With recent technologies a complete mission can be placed on a wafer including, power from an embedded radio nuclear thermal generator (RTG), PV, laser communications, imaging, photon thrusters for attitude control and other sensors. As one example, a futuristic 200 MW laser array consisting of 1 - 10 kw meter scale sub elements with a 100m baseline can propel a 10 gram wafer scale spacecraft with a 3m laser sail to 60AU/Year. Directed energy propulsion of low-mass spacecraft gives us an opportunity to capture images of Alpha Centauri and its planets, detailed imaging of the cosmic microwave background, set up interstellar communications by using gravity lenses around nearby stars to boost signals from interstellar probes, and much more. This system offers a very large range of missions allowing hundreds of wafer scale payload launches per day to reach this cosmological data reservoir. Directed Energy Propulsion is the only current technology that can provide a near-term path to utilize our Sun's Gravity Lens.

Magnetic Field Fluctuations Observed in the Heliosheath and Interstellar Magnetic Field by Voyager 1 at 115.7-124.9 AU during 2011-2013

NASA Astrophysics Data System (ADS)

Burlaga, L. F.; Ness, N. F.; Florinski, V.; Heerikhuisen, J.

2014-09-01

We discuss microscale fluctuations of the hour averages of the magnetic field B observed on a scale of one day by Voyager 1 (V1) from 2011.0 to 2012.3143 (when it was within the distant heliosheath, where the average magnetic field strength langBrang = 0.17 nT) and during the interval from 2012.6503 to 2013.5855 (when it was within the interstellar plasma with langBrang = 0.47 nT). In both regions, the fluctuations were primarily compressive fluctuations, varying along the average B (≈T direction in RTN coordinates). In the heliosheath, the average of the daily standard deviations (SDs) of the compressive and transverse components of B were langSDcrang = 0.010 nT and langSDtrang <= 0.005 nT (which is the limit of the measurement). In the distant heliosheath langSDcrang/langBrang = 0.06, and the distributions of SD were skewed and highly kurtotic. The interstellar magnetic field (ISMF) strength was B = 0.48 nT, but the fluctuations were below the limit of measurement: langSDcrang = 0.004 nT and langabs(SDt)rang = 0.004 nT. The distributions of these interstellar SDs have skewness and kurtosis consistent with a Gaussian noise distribution. We also discuss the fluctuations of 48 s averages of B on a scale of 1 day during a 30 day interval when V1 was observing the ISMF. For the fluctuations in all three components of B, SD = 0.010 nT, which gives an upper limit on the fluctuations of the ISMF on the scales observed by V1. This SD rules out the possibility that there is significant power in electromagnetic fluctuations generated by pickup ion ring instabilities at these scales, which strongly constrains models of the IBEX ribbon.

A study of birefringence in the interstellar medium in the direction of the Crab Nebula

NASA Technical Reports Server (NTRS)

Martin, P. G.; Angel, J. R. P.

1974-01-01

The interstellar medium may be regarded as a weak wave plate, the linear birefringence arising from the aligned grains which produce interstellar linear polarization. Using the Crab Nebula as a background source of linearly polarized light we have investigated this birefringence by measurements of circular polarization. The circular component is found to vary with the intrinsic linear polarization in a sinusoidal fashion characteristic of a wave plate with the orientation expected from independent measurements of the interstellar linear polarization in the same direction. Measurements of the wavelength dependence, together with the sense and magnitude of the circular polarization are interpreted as evidence for the dielectric nature of the interstellar grain materials. These observations provide a firm basis for a similar interpretation of the circular polarization of reddened stars. The observations of the stars can then be used to study the grain composition and the structure of the magnetic field in many directions in the Galaxy.

The VMC Survey. XXIX. Turbulence-controlled Hierarchical Star Formation in the Small Magellanic Cloud

NASA Astrophysics Data System (ADS)

Sun, Ning-Chen; de Grijs, Richard; Cioni, Maria-Rosa L.; Rubele, Stefano; Subramanian, Smitha; van Loon, Jacco Th.; Bekki, Kenji; Bell, Cameron P. M.; Ivanov, Valentin D.; Marconi, Marcella; Muraveva, Tatiana; Oliveira, Joana M.; Ripepi, Vincenzo

2018-05-01

In this paper we report a clustering analysis of upper main-sequence stars in the Small Magellanic Cloud, using data from the VMC survey (the VISTA near-infrared YJK s survey of the Magellanic system). Young stellar structures are identified as surface overdensities on a range of significance levels. They are found to be organized in a hierarchical pattern, such that larger structures at lower significance levels contain smaller ones at higher significance levels. They have very irregular morphologies, with a perimeter–area dimension of 1.44 ± 0.02 for their projected boundaries. They have a power-law mass–size relation, power-law size/mass distributions, and a log-normal surface density distribution. We derive a projected fractal dimension of 1.48 ± 0.03 from the mass–size relation, or of 1.4 ± 0.1 from the size distribution, reflecting significant lumpiness of the young stellar structures. These properties are remarkably similar to those of a turbulent interstellar medium, supporting a scenario of hierarchical star formation regulated by supersonic turbulence.

RadioAstron space-ground interferometer at 324 MHz: interstellar plasma in the direction to PSR B1919+21

NASA Astrophysics Data System (ADS)

Smirnova, Tatiana; Andrianov, Andrey; Shishov, Vladimir

We present results obtained from analysis of our observations carried out on the space-ground interferometer RadioAstron at frequency of 324 MHz. Observations were conducted on 04.07.12 with a baseline projection of 60000 km. We used two ground telescopes: GBT and WSRT with the space radio telescope (SRT). Notable visibility amplitudes were detected at all baseline projections. We found that frequency structure of interstellar scintillation for pulsar B1919+21 is defined by angular refraction with refractive angle: θref ≈ 60 mas. Cosmic prism is located near to observer with a distance of about 10 pc. The scattering angle is resolved by cosmic interferometer and θsc is about 1 mas. The temporal structure of scintillation is mainly defined by interstellar plasma of extended media, but frequency structure - by extended media and prism influence. We detected also the influence of ionosphere on a phase variations of interferometer.

Galactic gamma-ray observations and galactic structure

NASA Technical Reports Server (NTRS)

Stecker, F. W.

1975-01-01

Recent observations of gamma-rays originating in the galactic disk together with radio observations, support an emerging picture of the overall structure of our galaxy with higher interstellar gas densities and star formation rates in a region which corresponds to that of the inner arms. The emerging picture is one where molecular clouds make up the dominant constituent of the interstellar gas in the inner galaxy and play a key role in accounting for the gamma-rays and phenomena associated with the production of young stars and other population 1 objects. In this picture, cosmic rays are associated with supernovae and are primarily of galactic origin. These newly observed phenomena can be understood as consequences of the density wave theories of spiral structure. Based on these new developments, the suggestion is made that a new galactic population class, Population O, be added to the standard Populations 1 and 2 in order to recognize important differences in dynamics and distribution between diffuse galactic H1 and interstellar molecular clouds.

Interaction of the jet from the neutron star with the interstellar medium

NASA Astrophysics Data System (ADS)

Kiikov, S. O.

2017-12-01

The interaction between the hypersonic plasma jet from the accreting neutron star and the ambient interstellar medium is studied. It is assumed that the jet is launched from the accretion disk via the open magnetic field anchored in the disk. The analytical investigation for the structure of the working surface of the jet is carried out. The estimates of the volume stream functions in the region of the interaction between the jet and the interstellar medium are derived. The obtained results allow to examine the distribution of the plasma velocity fields in the interaction region.

Interstellar gas in the Gum Nebula

NASA Technical Reports Server (NTRS)

Wallerstein, G.; Jenkins, E. B.; Silk, J.

1980-01-01

A survey of the interstellar gas near the Gum Nebula by optical observation of 67 stars at Ca II, 42 stars at Na I, and 14 stars in the UV with the Copernicus satellite provided radial velocities and column densities for all resolved absorption components. Velocity dispersions for gas in the Gum Nebula are not significantly larger than in the general interstellar medium; the ionization structure is predominantly that of an H II region with moderately high ionization. Denser, more highly ionized clouds are concentrated toward the Gum Nebula; these clouds do not show the anomalously high ionization observed in the Vela remnant clouds.

Measurements of anisotropy in the cosmic microwave background radiation at 0.5 deg angular scales near the star gamma ursae minoris

NASA Technical Reports Server (NTRS)

Devlin, M. J.; Clapp, A. C.; Gundersen, J. O.; Hagmann, C. A.; Hristov, V. V.; Lange, A. E.; Lim, M. A.; Lubin, P. M.; Mauskopf, P. D.; Meinhold, P. R.

1994-01-01

We present results from a four-frequency observation of a 6 deg x 0.6 deg strip of the sky centered near the star Gamma Ursae Minoris (GUM) during the fourth flight of the Millimeter-wave Anistropy experiment(MAX). The observation was made with a 1.4 deg peak-to-peak sinusoidal chop in all bands. The FWHM beam sizes were 0.55 deg +/- 0.05 deg at 3.5 per cm and 0.75 deg +/- 0.05 deg at 6, 9, and 14 per cm. During this observation significant correlated structure was observed at 3.5, 6 and 9 per cm with amplitudes similar to those observed in the GUM region during the second and third fligts of MAX. The frequency spectrum is consistent with cosmic microwave background (CMB) and inconsistent with thermal emission from interstellar dust. The extrapolated amplitudes of synchrotron and free-free emission are too small to account for the amplitude of the observed structure, If all of the structure is attributed to CMB anisotropy with a Gaussian autocorrelation function and a coherence angle of 25 min, then the most probable values of delta T/T(sub CMB) in the 3.5, 6 and 9 per cm bads are (4.3 +2.7/-1.6) x 10(exp -5), 2.8 (+4.3/-1/1) x 10(exp -5), and 3.5 (+3.0/-1.6) x 10(exp -5) (95% confidence upper and lower limits), respectively.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Opher, M.; Drake, J. F.; Zieger, B.

The classic accepted view of the heliosphere is a quiescent, comet-like shape aligned in the direction of the Sun’s travel through the interstellar medium (ISM) extending for thousands of astronomical units (AUs). Here, we show, based on magnetohydrodynamic (MHD) simulations, that the tension (hoop) force of the twisted magnetic field of the Sun confines the solar wind plasma beyond the termination shock and drives jets to the north and south very much like astrophysical jets. These jets are deflected into the tail region by the motion of the Sun through the ISM similar to bent galactic jets moving through themore » intergalactic medium. The interstellar wind blows the two jets into the tail but is not strong enough to force the lobes into a single comet-like tail, as happens to some astrophysical jets. Instead, the interstellar wind flows around the heliosphere and into the equatorial region between the two jets. As in some astrophysical jets that are kink unstable, we show here that the heliospheric jets are turbulent (due to large-scale MHD instabilities and reconnection) and strongly mix the solar wind with the ISM beyond 400 AU. The resulting turbulence has important implications for particle acceleration in the heliosphere. The two-lobe structure is consistent with the energetic neutral atom (ENA) images of the heliotail from IBEX where two lobes are visible in the north and south and the suggestion from the Cassini ENAs that the heliosphere is “tailless.”.« less

Simulation of optical interstellar scintillation

NASA Astrophysics Data System (ADS)

Habibi, F.; Moniez, M.; Ansari, R.; Rahvar, S.

2013-04-01

Aims: Stars twinkle because their light propagates through the atmosphere. The same phenomenon is expected on a longer time scale when the light of remote stars crosses an interstellar turbulent molecular cloud, but it has never been observed at optical wavelengths. The aim of the study described in this paper is to fully simulate the scintillation process, starting from the molecular cloud description as a fractal object, ending with the simulations of fluctuating stellar light curves. Methods: Fast Fourier transforms are first used to simulate fractal clouds. Then, the illumination pattern resulting from the crossing of background star light through these refractive clouds is calculated from a Fresnel integral that also uses fast Fourier transform techniques. Regularisation procedure and computing limitations are discussed, along with the effect of spatial and temporal coherency (source size and wavelength passband). Results: We quantify the expected modulation index of stellar light curves as a function of the turbulence strength - characterised by the diffraction radius Rdiff - and the projected source size, introduce the timing aspects, and establish connections between the light curve observables and the refractive cloud. We extend our discussion to clouds with different structure functions from Kolmogorov-type turbulence. Conclusions: Our study confirms that current telescopes of ~4 m with fast-readout, wide-field detectors have the capability of discovering the first interstellar optical scintillation effects. We also show that this effect should be unambiguously distinguished from any other type of variability through the observation of desynchronised light curves, simultaneously measured by two distant telescopes.

Tentative Identification of Interstellar Dust in the Magnetic Wall of the Heliosphere

NASA Astrophysics Data System (ADS)

Frisch, P. C.

2006-06-01

Data showing that light from nearby stars, <40 pc, is weakly polarized are consistent with the capture and alignment of dust grains in the magnetic wall of the heliosphere. These data, from Tinbergen (1982) and Piirola (1977), were acquired during the solar minimum of the mid-1970's when the magnetic wall was expected to form at negative ecliptic latitudes because the solar magnetic polarity was north-pole-positive. The polarization is seen primarily at negative ecliptic latitudes, consistent with the expected magnetic wall position. The interstellar magnetic field direction at the Sun is derived from these data. The small dust grains most likely to cause the polarization are also the grains excluded from the heliosphere by small gyroradii, <100 AU. The direction of maximum polarization is offset by ˜ 20 --40 deg. from the inflow direction of the large grains that are gravitationally focused in the heliosphere tail. Interstellar dust grains in and near the heliosphere form a potential contaminant of the cosmic microwave background signal, which should then be identifiable because the spatial behavior of these grains depends on the phase of the 22 year solar magnetic activity cycle. The author would like to thank NASA for supporting her research.

Potential interstellar noble gas molecules: ArOH+ and NeOH+ rovibrational analysis from quantum chemical quartic force fields

NASA Astrophysics Data System (ADS)

Theis, Riley A.; Fortenberry, Ryan C.

2016-03-01

The discovery of ArH+ in the interstellar medium has shown that noble gas chemistry may be of more chemical significance than previously believed. The present work extends the known chemistry of small noble gas molecules to NeOH+ and ArOH+. Besides their respective neonium and argonium diatomic cation cousins, these hydroxyl cation molecules are the most stable small noble gas molecules analyzed of late. ArOH+ is once again more stable than the neon cation, but both are well-behaved enough for a complete quartic force field analysis of their rovibrational properties. The Ar-O bond in ArOH+ , for instance, is roughly three-quarters of the strength of the Ar-H bond in ArH+ highlighting the rigidity of this system. The rotational constants, geometries, and vibrational frequencies for both molecules and their various isotopologues are computed from ab initio quantum chemical theory at high-level, and it is shown that these cations may form in regions where peroxy or weakly-bound alcohols may be present. The resulting data should be of significant assistance for the laboratory or observational analysis of these potential interstellar molecules.

The Astrobiological Case for Our Cosmic Ancestry

NASA Astrophysics Data System (ADS)

Wickramasinghe, Chandra

With steadily mounting evidence that points to a cosmic origin of terrestrial life, a cultural barrier prevails against admitting that such a connection exists. Astronomy continues to reveal the presence of organic molecules and organic dust on a huge cosmic scale, amounting to a third of interstellar carbon tied up in this form. Just as the overwhelming bulk of organics on Earth stored over geological timescales are derived from the degradation of living cells, so it seems most likely that interstellar organics in large measure also derive from biology. As we enter a new decade -- the year 2010 -- a clear pronouncement of our likely alien ancestry and of the existence of extraterrestrial life on a cosmic scale would seem to be overdue.

The astrobiological case for our cosmic ancestry

NASA Astrophysics Data System (ADS)

Wickramasinghe, Chandra

2010-04-01

With steadily mounting evidence that points to a cosmic origin of terrestrial life, a cultural barrier prevails against admitting that such a connection exists. Astronomy continues to reveal the presence of organic molecules and organic dust on a huge cosmic scale, amounting to a third of interstellar carbon tied up in this form. Just as the overwhelming bulk of organics on Earth stored over geological timescales are derived from the degradation of living cells, so it seems likely that interstellar organics in large measure also derive from biology. As we enter a new decade - the year 2010 - a clear pronouncement of our likely alien ancestry and of the existence of extraterrestrial life on a cosmic scale would seem to be overdue.

Optical Polarization as a Probe of the Local Interstellar Medium

NASA Technical Reports Server (NTRS)

Tinbergen, J.

1984-01-01

The use of interstellar polarization as a tool for measuring interstellar dust is discussed. Problems resulting from dust and magnetic field configurations becoming mixed up are discussed, as is the availability of sufficiently bright stars to obtain the photons needed for precision measurements. It is proposed that: (1) on the scale of several hundred parsec, there is a preferential magnetic field direction, as evidenced by observations at the Galactic poles and selected longitudes in the Galactic plane; (2) the local (r 50 pc) region is devoid of dust, as evidenced by the mean square degree of polarization as a function of distance; and, less certainly, that (3) at a distance of less than 5 pc, there is a patch of dust which may be of interest in connection with cloud models.

Maximum number of habitable planets at the time of Earth's origin: new hints for panspermia?

PubMed

von Bloh, Werner; Franck, Siegfried; Bounama, Christine; Schellnhuber, Hans-Joachim

2003-04-01

New discoveries have fuelled the ongoing discussion of panspermia, i.e. the transport of life from one planet to another within the solar system (interplanetary panspermia) or even between different planetary systems (interstellar panspermia). The main factor for the probability of interstellar panspermia is the average density of stellar systems containing habitable planets. The combination of recent results for the formation rate of Earth-like planets with our estimations of extrasolar habitable zones allows us to determine the number of habitable planets in the Milky Way over cosmological time scales. We find that there was a maximum number of habitable planets around the time of Earth's origin. If at all, interstellar panspermia was most probable at that time and may have kick-started life on our planet.

Infrared emission from hydrogenated amorphous carbon and amorphous carbon grains in the interstellar medium

NASA Technical Reports Server (NTRS)

Duley, W. W.; Jones, A. P.; Taylor, S. D.; Williams, D. A.

1993-01-01

The correlations deduced by Boulanger et al. (1990) from IRAS maps of the Chamaeleon, Taurus and Ursa Major molecular cloud complexes are interpreted in terms of the evolutionary hydrogenated amorphous carbon model of interstellar dust. In particular, regions of relatively strong 12-micron emission may be regions where recently accreted carbon is being converted by ambient UV to small PAHs in situ. Regions of weak 12-micron emission are probably quiescent regions where carbon has been annealed to amorphous carbon. Observational consequences of these inferences are briefly described.

On diamond, graphitic and amorphous carbons in primitive extraterrestrial solar system materials

NASA Technical Reports Server (NTRS)

Rietmeijer, Frans J. M.

1990-01-01

Carbon is among the most abundant elements in the universe and carbon chemistry in meteorites and comets is an important key to understanding many Solar System and interstellar processes. Yet, the mineralogical properties and interrelations between various structural forms of elemental carbon remain ambiguous. Crystalline elemental carbons include rhombohedral graphite, hexagonal graphite, cubic diamond, hexagonal diamond (i.e., lonsdaleite or carbon-2H) and chaoite. Elemental carbon also occurs as amorphous carbon and poorly graphitized (or turbostratic) carbon but of all the forms of elemental carbon only graphite is stable under physical conditions that prevail in small Solar System bodies and in the interstellar medium. The recent discovery of cubic diamond in carbonaceous chondrites and hexagonal diamond in chondritic interplanetary dust particles (IDPs) have created a renewed interest in the crystalline elemental carbons that were not formed by shock processes on a parent body. Another technique, Raman spectroscopy, confirms a widespread occurrence of disordered graphite in the Allende carbonaceous chondrite and in chondritic IDPs. Elemental carbons have also been identified by their characteristic K-edge features in electron energy loss spectra (EELS). However, the spectroscopic data do not necessarily coincide with those obtained by selected area electron diffraction (SAED). In order to interpret these data in terms of rational crystalline structures, it may be useful to consider the principles underlying electron diffraction and spectroscopic analyses. Electron diffraction depends on electron scattering, on the type of atom and the distance between atoms in a crystal lattice. Spectroscopic data are a function of the type of atom and the energy of bonds between atoms. Also, SAED is a bulk sampling technique when compared to techniques such as Raman spectroscopy or EELS. Thus, it appears that combined analyses provide contradictory results and that amorphous, or short-range ordered, carbon identified by conventional TEM imaging and SAED may show evidence for sp(3) bonds in EELS spectra. It is suggested that complex, nanometer-scale, mineralogical interrelations are common to all elemental carbons irrespective of their origin. The subsequent thermal history, or energy balance, will determine the ultimate microstructure.

Swift heavy ion irradiation of interstellar dust analogues. Small carbonaceous species released by cosmic rays

NASA Astrophysics Data System (ADS)

Dartois, E.; Chabot, M.; Pino, T.; Béroff, K.; Godard, M.; Severin, D.; Bender, M.; Trautmann, C.

2017-03-01

Context. Interstellar dust grain particles are immersed in vacuum ultraviolet (VUV) and cosmic ray radiation environments influencing their physicochemical composition. Owing to the energetic ionizing interactions, carbonaceous dust particles release fragments that have direct impact on the gas phase chemistry. Aims: The exposure of carbonaceous dust analogues to cosmic rays is simulated in the laboratory by irradiating films of hydrogenated amorphous carbon interstellar analogues with energetic ions. New species formed and released into the gas phase are explored. Methods: Thin carbonaceous interstellar dust analogues were irradiated with gold (950 MeV), xenon (630 MeV), and carbon (43 MeV) ions at the GSI UNILAC accelerator. The evolution of the dust analogues is monitored in situ as a function of fluence at 40, 100, and 300 K. Effects on the solid phase are studied by means of infrared spectroscopy complemented by simultaneously recording mass spectrometry of species released into the gas phase. Results: Specific species produced and released under the ion beam are analyzed. Cross sections derived from ion-solid interaction processes are implemented in an astrophysical context.

Rotation measure synthesis at the 2 m wavelength of the FAN region: unveiling screens and bubbles

NASA Astrophysics Data System (ADS)

Iacobelli, M.; Haverkorn, M.; Katgert, P.

2013-01-01

Context. Rotation measure synthesis of the Westerbork Synthesis Radio Telescope (WSRT) observations at λ ~ 2 m of the FAN region at l = 137°, b = +7° shows the morphology of structures in the ionized interstellar medium. Aims: We interpret the diffuse polarized synchrotron emission in terms of coherent structures in the interstellar medium and the properties of the interstellar magnetic field. Methods: We performed statistical analysis of the polarization data cube obtained through rotation measure synthesis. For the first time, cross-correlation is applied to identify and characterize polarized structures in Faraday depth space. Complementary information about the medium are derived from Hα emission, properties of nearby pulsars, and optical polarized starlight measurements. Results: We find an overall asymmetric Faraday dispersion function in a Faraday depth range of [-13, +5] rad m-2, which is peaked around -1 rad m-2. Three morphological patterns are recognized, showing structures on scales from degrees down to the beam size. The first structure is a nearby synchrotron emission component with low Faraday depth, filling the entire field of view. The second pattern is a circular polarization structure with enhanced (negative) Faraday depth, which has the same morphology as a low-emission region within the third component. This third component is interpreted as the background in which the circular structure is embedded. At low Faraday depth values, a low gradient across the imaged field is detected, almost aligned with the Galactic plane. Power spectra of polarized structures in Faraday depth space provide evidence of turbulence. Conclusions: A sign reversal in Faraday depth from the nearby component to the circular component indicates a reversal of the magnetic field component along the line of sight, from towards the observer and nearby to away from the observer at large distances. The distance to the nearby, extended component is estimated as ≲100 pc, which suggests that this structure corresponds to the Local Bubble wall. For the circular component, various physical interpretations are discussed. The most likely explanation is that the circular component seems to be the presence of a nearby (~200 pc away) relic Strömgren sphere, associated with an old unidentified white dwarf star and expanding in a low-density environment. Faraday rotation datacubes are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/549/A56

Time scales for molecule formation by ion-molecule reactions

NASA Technical Reports Server (NTRS)

Langer, W. D.; Glassgold, A. E.

1976-01-01

Analytical solutions are obtained for nonlinear differential equations governing the time-dependence of molecular abundances in interstellar clouds. Three gas-phase reaction schemes are considered separately for the regions where each dominates. The particular case of CO, and closely related members of the Oh and CH families of molecules, is studied for given values of temperature, density, and the radiation field. Nonlinear effects and couplings with particular ions are found to be important. The time scales for CO formation range from 100,000 to a few million years, depending on the chemistry and regime. The time required for essentially complete conversion of C(+) to CO in the region where the H3(+) chemistry dominates is several million years. Because this time is longer than or comparable to dynamical time scales for dense interstellar clouds, steady-state abundances may not be observed in such clouds.

Two-component scattering model and the electron density spectrum

NASA Astrophysics Data System (ADS)

Zhou, A. Z.; Tan, J. Y.; Esamdin, A.; Wu, X. J.

2010-02-01

In this paper, we discuss a rigorous treatment of the refractive scintillation caused by a two-component interstellar scattering medium and a Kolmogorov form of density spectrum. It is assumed that the interstellar scattering medium is composed of a thin-screen interstellar medium (ISM) and an extended interstellar medium. We consider the case that the scattering of the thin screen concentrates in a thin layer represented by a δ function distribution and that the scattering density of the extended irregular medium satisfies the Gaussian distribution. We investigate and develop equations for the flux density structure function corresponding to this two-component ISM geometry in the scattering density distribution and compare our result with the observations. We conclude that the refractive scintillation caused by this two-component ISM scattering gives a more satisfactory explanation for the observed flux density variation than does the single extended medium model. The level of refractive scintillation is strongly sensitive to the distribution of scattering material along the line of sight (LOS). The theoretical modulation indices are comparatively less sensitive to the scattering strength of the thin-screen medium, but they critically depend on the distance from the observer to the thin screen. The logarithmic slope of the structure function is sensitive to the scattering strength of the thin-screen medium, but is relatively insensitive to the thin-screen location. Therefore, the proposed model can be applied to interpret the structure functions of flux density observed in pulsar PSR B2111 + 46 and PSR B0136 + 57. The result suggests that the medium consists of a discontinuous distribution of plasma turbulence embedded in the interstellar medium. Thus our work provides some insight into the distribution of the scattering along the LOS to the pulsar PSR B2111 + 46 and PSR B0136 + 57.

The relationship between the galactic matter distribution, cosmic ray dynamics, and gamma ray production

NASA Technical Reports Server (NTRS)

Kniffen, D. A.; Fichtel, C. E.; Thompson, D. J.

1976-01-01

Theoretical considerations and analysis of the results of gamma ray astronomy suggest that the galactic cosmic rays are dynamically coupled to the interstellar matter through the magnetic fields, and hence the cosmic ray density should be enhanced where the matter density is greatest on the scale of galactic arms. This concept has been explored in a galactic model using recent 21 cm radio observations of the neutral hydrogen and 2.6 mm observations of carbon monoxide, which is considered to be a tracer of molecular hydrogen. The model assumes: (1) cosmic rays are galactic and not universal; (2) on the scale of galactic arms, the cosmic ray column (surface) density is proportional to the total interstellar gas column density; (3) the cosmic ray scale height is significantly larger than the scale height of the matter; and (4) ours is a spiral galaxy characterized by an arm to interarm density ratio of about 3:1.

Self-regulating galaxy formation. Part 1: HII disk and Lyman alpha pressure

NASA Technical Reports Server (NTRS)

Cox, D. P.

1983-01-01

Assuming a simple but physically based prototype for behavior of interstellar material during formation of a disk galaxy, coupled with the lowest order description of infall, a scenario is developed for self-regulated disk galaxy formation. Radiation pressure, particularly that of Lyman depha (from fluorescence conversion Lyman continuum), is an essential component, maintaining an inflated disk and stopping infall when only a small fraction of the overall perturbation has joined the disk. The resulting galaxies consist of a two dimensional family whose typical scales and surface density are expressable in terms of fundamental constants. The model leads naturally to galaxies with a rich circumgalactic environment and flat rotation curves (but is weak in its analysis of the subsequent evolution of halo material).

Synthesis and chirality of amino acids under interstellar conditions.

PubMed

Giri, Chaitanya; Goesmann, Fred; Meinert, Cornelia; Evans, Amanda C; Meierhenrich, Uwe J

2013-01-01

Amino acids are the fundamental building blocks of proteins, the biomolecules that provide cellular structure and function in all living organisms. A majority of amino acids utilized within living systems possess pre-specified orientation geometry (chirality); however the original source for this specific orientation remains uncertain. In order to trace the chemical evolution of life, an appreciation of the synthetic and evolutional origins of the first chiral amino acids must first be gained. Given that the amino acids in our universe are likely to have been synthesized in molecular clouds in interstellar space, it is necessary to understand where and how the first synthesis might have occurred. The asymmetry of the original amino acid synthesis was probably the result of exposure to chiral photons in the form of circularly polarized light (CPL), which has been detected in interstellar molecular clouds. This chirality transfer event, from photons to amino acids, has been successfully recreated experimentally and is likely a combination of both asymmetric synthesis and enantioselective photolysis. A series of innovative studies have reported successful simulation of these environments and afforded production of chiral amino acids under realistic circumstellar and interstellar conditions: irradiation of interstellar ice analogues (CO, CO2, NH3, CH3OH, and H2O) with circularly polarized ultraviolet photons at low temperatures does result in enantiomer enriched amino acid structures (up to 1.3% ee). This topical review summarizes current knowledge and recent discoveries about the simulated interstellar environments within which amino acids were probably formed. A synopsis of the COSAC experiment onboard the ESA cometary mission ROSETTA concludes this review: the ROSETTA mission will soft-land on the nucleus of the comet 67P/Churyumov-Gerasimenko in November 2014, anticipating the first in situ detection of asymmetric organic molecules in cometary ices.

Grain Spectroscopy

NASA Technical Reports Server (NTRS)

Allamandola, L. J.

1992-01-01

Our fundamental knowledge of interstellar grain composition has grown substantially during the past two decades thanks to significant advances in two areas: astronomical infrared spectroscopy and laboratory astrophysics. The opening of the mid-infrared, the spectral range from 4000-400 cm(sup -1) (2.5-25 microns), to spectroscopic study has been critical to this progress because spectroscopy in this region reveals more about a materials molecular composition and structure than any other physical property. Infrared spectra which are diagnostic of interstellar grain composition fall into two categories: absorption spectra of the dense and diffuse interstellar media, and emission spectra from UV-Vis rich dusty regions. The former will be presented in some detail, with the latter only very briefly mentioned. This paper summarized what we have learned from these spectra and presents 'doorway' references into the literature. Detailed reviews of many aspects of interstellar dust are given.

Ionization of Interstellar Hydrogen

NASA Astrophysics Data System (ADS)

Whang, Y. C.

1996-09-01

Interstellar hydrogen can penetrate through the heliopause, enter the heliosphere, and may become ionized by photoionization and by charge exchange with solar wind protons. A fluid model is introduced to study the flow of interstellar hydrogen in the heliosphere. The flow is governed by moment equations obtained from integration of the Boltzmann equation over the velocity space. Under the assumption that the flow is steady axisymmetric and the pressure is isotropic, we develop a method of solution for this fluid model. This model and the method of solution can be used to study the flow of neutral hydrogen with various forms of ionization rate β and boundary conditions for the flow on the upwind side. We study the solution of a special case in which the ionization rate β is inversely proportional to R2 and the interstellar hydrogen flow is uniform at infinity on the upwind side. We solve the moment equations directly for the normalized density NH/NN∞, bulk velocity VH/VN∞, and temperature TH/TN∞ of interstellar hydrogen as functions of r/λ and z/λ, where λ is the ionization scale length. The solution is compared with the kinetic theory solution of Lallement et al. The fluid solution is much less time-consuming than the kinetic theory solutions. Since the ionization rate for production of pickup protons is directly proportional to the local density of neutral hydrogen, the high-resolution solution of interstellar neutral hydrogen obtained here will be used to study the global distribution of pickup protons.

Assessment of the Polycyclic Aromatic Hydrocarbon-Diffuse Interstellar Band Proposal

NASA Technical Reports Server (NTRS)

Salama, Farid; Bakes, F.; Allamandola, L.; Tielens, A. G. G. M.; Witteborn, Fred C. (Technical Monitor)

1995-01-01

The potential link between neutral and/or ionized polycyclic aromatic hydrocarbons (PAHs) and the diffuse interstellar band (DIB) carriers is examined. Based on the study of the general physical and chemical properties of PAHs, an assessment is made of their possible contribution to the DIB carriers. It is found that, under the conditions reigning in the diffuse interstellar medium, PAHs can be present in the form of neutral molecules as well as positive and/or negative ions. The charge distribution of small PAHs is dominated, however, by two charge states at one time with compact PAHs present only in the neutral and cationic forms. Each PAH has a distinct spectral signature depending on its charge state. Moreover, the spectra of ionized PAHs are always clearly dominated by a single band in the DIB spectral range. In the case of compact PAH ions, the strongest absorption band is of type A (i.e., the band is broad, falls in the high energy range of the spectrum, and possess a large oscillator strength), and seems to correlate with strong and broad DIBs. In the case of non-compact PAH ions, the strongest absorption band is of type I (i.e., the band is narrow, falls in the low energy range of the spectrum, and possess a small oscillator strength), and seems to correlate with weak and narrow DIBs. Potential molecular size and structure constraints for interstellar PAHs are derived by comparing known DIB characteristics to the spectroscopic properties of PAHs. It is found that: (i) Only neutral PAHs larger than about 30 carbon atoms could, if present, contribute to the DIBs. (ii) For compact PAHs, only ions with less than about 250 carbon atoms could, if present, contribute to the DIBs. (iii) The observed distribution of the DIBs between strong/moderate and broad bands on the one hand and weak and narrow bands on the other hand can easily be interpreted in the context of the PAH proposal by a distribution between compact and non-compact PAH ions, respectively. A plausible correlation between PAH charge states and DIB "families" is thus provided by the PAH-DIB proposal. Following this proposal, DIB families would provide tracers of conditions within a cloud which globally determine the relative importance of cations, anions, and neutral species, rather than tracers of a specific species. Observational predictions are given to establish the viability of the PAH hypothesis. It is concluded that small PAH ions are very promising candidates as DIB carriers provided their population is dominated by a finite number (100-200) of species. A key test for the PAH proposal, consisting of laboratory and astronomical investigations of the ultraviolet range, is called for.

Assessment of the polycyclic aromatic hydrocarbon-diffuse interstellar band proposal

NASA Technical Reports Server (NTRS)

Salama, F.; Bakes, E. L.; Allamandola, L. J.; Tielens, A. G.

1996-01-01

The potential link between neutral and/or ionized polycyclic aromatic hydrocarbons (PAHs) and the diffuse interstellar band (DIB) carriers is examined. Based on the study of the general physical and chemical properties of PAHs, an assessment is made of their possible contribution to the DIB carriers. It is found that, under the conditions reigning in the diffuse interstellar medium, PAHs can be present in the form of neutral molecules as well as positive and/or negative ions. The charge distribution of small PAHs is dominated, however, by two charge states at one time with compact PAHs present only in the neutral and cationic forms. Each PAH has a distinct spectral signature depending on its charge state. Moreover, the spectra of ionized PAHs are always clearly dominated by a single band in the DIB spectral range. In the case of compact PAH ions, the strongest absorption band is of type A (i.e., the band is broad, falls in the high-energy range of the spectrum, and possesses a large oscillator strength), and seems to correlate with strong and broad DIBs. For noncompact PAH ions, the strongest absorption band is of type I (i.e., the band is narrow, falls in the low-energy range of the spectrum, and possesses a small oscillator strength), and seems to correlate with weak and narrow DIBs. Potential molecular size and structure constraints for interstellar PAHs are derived by comparing known DIB characteristics to the spectroscopic properties of PAHs. It is found that (i) only neutral PAHs larger than about 30 carbon atoms could, if present, contribute to the DIBs. (ii) For compact PAHs, only ions with less than about 250 carbon atoms could, if present, contribute to the DIBs. (iii) The observed distribution of the DIBs between strong/moderate and broad bands on the one hand and weak and narow bands on the other can easily be interpreted in the context of the PAH proposal by a distribution of compact and noncompact PAH ions, respectively. A plausible correlation between PAH charge states and DIB "families" is thus provided by the PAH-DIB proposal. Following this proposal, DIB families would reflect conditions within a cloud which locally determine the relative importance of cations, anions, and neutral species, rather than tracers of a specific species. Observational predictions are given to establish the viability of the PAH hypothesis. It is concluded that small PAH ions are very promising candidates as DIB carriers provided their population is dominated by a finite number (100-200) of species. A key test for the PAH proposal, consisting of laboratory and astronomical investigations in the ultraviolet range, is called for.

The demography of extraterrestrial civilizations

NASA Technical Reports Server (NTRS)

Billingham, J.

1981-01-01

Studies carried out within the last ten years on the nature and distribution of extraterrestrial intelligent life are reviewed. Arguments for the absence of intelligent life in the Galaxy based on the assumption that at least some of these would have engaged in colonization and for the presence of colonies of extraterrestrials in some undiscovered location in the solar system are presented, and it is noted that both these views rest on the notion that interstellar travel can be achieved at high velocities in very large vehicles, which has been questioned. Alternative suggestions concerning interstellar exploration by automated probes and the possible extended time scale and motivation for galactic colonization are pointed out. Attention is then given to arguments for the extreme smallness of one of the factors in the Drake equation used to estimate the number of communicative extraterrestrial civilizations in the Galaxy, including the frequency of single stars, the likelihood that planets with the correct initial composition and conditions for life are at the proper distance from their stars, the probability of the formation of DNA and the origin of life, and the time for the evolution of intelligence. It is concluded that it seems likely that other civilizations exist in the Galaxy, although the number and distribution of such civilizations may only be determined by the detection of one or more examples.

Physical state of interstellar atoms. [from Copernicus satellite UV data

NASA Technical Reports Server (NTRS)

York, D. G.

1974-01-01

Brief survey of the physical conditions along the lines of sight to reddened and unreddened stars, as determined from Copernicus observation of interstellar lines between 95 and 300 nm. Differences in ionization structure and density between clouds and the local intercloud medium are discussed. Some new data for beta Centauri is used to supplement the previously available data.

Interstellar Matter

NASA Astrophysics Data System (ADS)

Savage, B.; Murdin, P.

2000-11-01

The enormous volume of space between the stars in the Milky Way Galaxy is filled with interstellar matter (ISM). The ISM plays a central role in the processes of STAR FORMATION and GALAXY EVOLUTION. Stars form from the ISM in dense molecular clouds. The radiant and mechanical energy produced by stars heats, ionizes, and produces structures in the ISM. Gradual or catastrophic mass loss from stars ...

Project Icarus: Stakeholder Scenarios for an Interstellar Exploration Program

NASA Astrophysics Data System (ADS)

Hein, A. M.; Tziolas, A. C.; Osborne, R.

The Project Icarus Study Group's objective is to design a mainly fusion-propelled interstellar probe. The starting point are the results of the Daedalus study, which was conducted by the British Interplanetary Society during the 1970's. As the Daedalus study already indicated, interstellar probes will be the result of a large scale, decade-long development program. To sustain a program over such long periods, the commitment of key stakeholders is vital. Although previous publications identified political and societal preconditions to an interstellar exploration program, there is a lack of more specific scientific and political stakeholder scenarios. This paper develops stakeholder scenarios which allow for a more detailed sustainability assessment of future programs. For this purpose, key stakeholder groups and their needs are identified and scientific and political scenarios derived. Political scenarios are based on patterns of past space programs but unprecedented scenarios are considered as well. Although it is very difficult to sustain an interstellar exploration program, there are scenarios in which this seems to be possible, e.g. the discovery of life within the solar system and on an exoplanet, a global technology development program, and dual-use of technologies for defence and security purposes. This is a submission of the Project Icarus Study Group.

From Interstellar PAHs and Ices to the Origin of Life

NASA Technical Reports Server (NTRS)

Allamandola, Louis J.; DeVincenzi, Donald L. (Technical Monitor)

2000-01-01

Tremendous strides have been made in our understanding of interstellar material over the past twenty years thanks to significant, parallel developments in observational astronomy and laboratory astrophysics. Twenty years ago the composition of interstellar dust was largely guessed at, the concept of ices in dense molecular clouds ignored, and the notion of large, abundant, gas phase, carbon rich molecules widespread throughout the interstellar medium (ISM) considered impossible. Today the composition of dust in the diffuse ISM is reasonably well constrained to micron-sized cold refractory materials comprised of amorphous and crystalline silicates mixed with an amorphous carbonaceous material containing aromatic structural units and short, branched aliphatic chains. In dense molecular clouds, the birthplace of stars and planets, these cold dust particles are coated with mixed molecular ices whose composition is very well constrained. Lastly, the signature of carbon-rich polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by earlier interstellar chemistry standards, is widespread throughout the Universe. The first part of this lecture will describe how infrared studies of interstellar space, combined with laboratory simulations, have revealed the composition of interstellar ices (the building blocks of comets) and the high abundance and nature of interstellar PAHs. The laboratory database has now enabled us to gain insight into the identities, concentrations, and physical state of many interstellar materials. Within a dense molecular cloud, and especially in the solar nebula during the star and planet formation stage, the materials frozen into interstellar/precometary ices are photoprocessed by ultraviolet light, producing more complex molecules. The remainder of the presentation will focus on the photochemical evolution of these materials and the possible role of these compounds on the early Earth. As these materials are thought to be the building blocks of comets and related to the carbonaceous components of micrometeorites, they are likely to have been important sources of complex organic materials on the early Earth and their composition may be related to the origin of life.

Chirality, photochemistry and the detection of amino acids in interstellar ice analogues and comets.

PubMed

Evans, Amanda C; Meinert, Cornelia; Giri, Chaitanya; Goesmann, Fred; Meierhenrich, Uwe J

2012-08-21

The primordial appearance of chiral amino acids was an essential component of the asymmetric evolution of life on Earth. In this tutorial review we will explore the original life-generating, symmetry-breaking event and summarise recent thoughts on the origin of enantiomeric excess in the universe. We will then highlight the transfer of asymmetry from chiral photons to racemic amino acids and elucidate current experimental data on the photochemical synthesis of amino and diamino acid structures in simulated interstellar and circumstellar ice environments. The chirality inherent within actual interstellar (cometary) ice environments will be considered in this discussion: in 2014 the Rosetta Lander Philae onboard the Rosetta space probe is planned to detach from the orbiter and soft-land on the surface of the nucleus of comet 67P/Churyumov-Gerasimenko. It is equipped for the in situ enantioselective analysis of chiral prebiotic organic species in cometary ices. The scientific design of this mission will therefore be presented in the context of analysing the formation of amino acid structures within interstellar ice analogues as a means towards furthering understanding of the origin of asymmetric biological molecules.

Mapping spiral structure on the far side of the Milky Way

NASA Astrophysics Data System (ADS)

Sanna, Alberto; Reid, Mark J.; Dame, Thomas M.; Menten, Karl M.; Brunthaler, Andreas

2017-10-01

Direct measurements of distances to astronomical sources rely on parallax, which can usually only be measured for relatively nearby objects. The far side of the Milky Way has been impossible to measure accurately, because the parallax is very small and interstellar dust blocks optical light from those regions. Sanna et al. used radio interferometry to directly determine the parallax distance to a star-forming region on the far side of the Galaxy. They also used a method of inferring distances from transverse motions, which produced the same answer. This allowed them to trace one of the Milky Way's spiral arms through almost an entire rotation.

Architecture Of A Sciencecraft To Fly Past Pluto

NASA Technical Reports Server (NTRS)

Price, Humphrey W.; Staehle, Robert L.; Alkalaj, Leon; Terrile, Richard J.; Miyake, Robert N.

1995-01-01

Two reports discuss architecture of proposed small sciencecraft carrying scientific instruments on trajectory passing near Pluto and continuing into interstellar space. Emphasizes those aspects of design pertaining to compactness, efficiency, and small mass (dry mass less than 100 kg). System block diagram of sciencecraft divided into blocks for sensors, integrated microelectronics, and motive effectors.

Modulation of Small-scale Turbulence Structure by Large-scale Motions in the Absence of Direct Energy Transfer.

NASA Astrophysics Data System (ADS)

Brasseur, James G.; Juneja, Anurag

1996-11-01

Previous DNS studies indicate that small-scale structure can be directly altered through ``distant'' dynamical interactions by energetic forcing of the large scales. To remove the possibility of stimulating energy transfer between the large- and small-scale motions in these long-range interactions, we here perturb the large scale structure without altering its energy content by suddenly altering only the phases of large-scale Fourier modes. Scale-dependent changes in turbulence structure appear as a non zero difference field between two simulations from identical initial conditions of isotropic decaying turbulence, one perturbed and one unperturbed. We find that the large-scale phase perturbations leave the evolution of the energy spectrum virtually unchanged relative to the unperturbed turbulence. The difference field, on the other hand, is strongly affected by the perturbation. Most importantly, the time scale τ characterizing the change in in turbulence structure at spatial scale r shortly after initiating a change in large-scale structure decreases with decreasing turbulence scale r. Thus, structural information is transferred directly from the large- to the smallest-scale motions in the absence of direct energy transfer---a long-range effect which cannot be explained by a linear mechanism such as rapid distortion theory. * Supported by ARO grant DAAL03-92-G-0117

Deep view of the Large Magellanic Cloud with six years of Fermi-LAT observations

NASA Astrophysics Data System (ADS)

Ackermann, M.; Albert, A.; Atwood, W. B.; Baldini, L.; Ballet, J.; Barbiellini, G.; Bastieri, D.; Bellazzini, R.; Bissaldi, E.; Bloom, E. D.; Bonino, R.; Brandt, T. J.; Bregeon, J.; Bruel, P.; Buehler, R.; Caliandro, G. A.; Cameron, R. A.; Caragiulo, M.; Caraveo, P. A.; Cavazzuti, E.; Cecchi, C.; Charles, E.; Chekhtman, A.; Chiang, J.; Chiaro, G.; Ciprini, S.; Cohen-Tanugi, J.; Cutini, S.; D'Ammando, F.; de Angelis, A.; de Palma, F.; Desiante, R.; Digel, S. W.; Drell, P. S.; Favuzzi, C.; Ferrara, E. C.; Focke, W. B.; Franckowiak, A.; Fusco, P.; Gargano, F.; Gasparrini, D.; Giglietto, N.; Giordano, F.; Godfrey, G.; Grenier, I. A.; Grondin, M.-H.; Guillemot, L.; Guiriec, S.; Harding, A. K.; Hill, A. B.; Horan, D.; Jóhannesson, G.; Knödlseder, J.; Kuss, M.; Larsson, S.; Latronico, L.; Li, J.; Li, L.; Longo, F.; Loparco, F.; Lubrano, P.; Maldera, S.; Martin, P.; Mayer, M.; Mazziotta, M. N.; Michelson, P. F.; Mizuno, T.; Monzani, M. E.; Morselli, A.; Murgia, S.; Nuss, E.; Ohsugi, T.; Orienti, M.; Orlando, E.; Ormes, J. F.; Paneque, D.; Pesce-Rollins, M.; Piron, F.; Pivato, G.; Porter, T. A.; Rainò, S.; Rando, R.; Razzano, M.; Reimer, A.; Reimer, O.; Romani, R. W.; Sánchez-Conde, M.; Schulz, A.; Sgrò, C.; Siskind, E. J.; Smith, D. A.; Spada, F.; Spandre, G.; Spinelli, P.; Suson, D. J.; Takahashi, H.; Thayer, J. B.; Tibaldo, L.; Torres, D. F.; Tosti, G.; Troja, E.; Vianello, G.; Wood, M.; Zimmer, S.

2016-02-01

Context. The nearby Large Magellanic Cloud (LMC) provides a rare opportunity of a spatially resolved view of an external star-forming galaxy in γ-rays. The LMC was detected at 0.1-100 GeV as an extended source with CGRO/EGRET and using early observations with the Fermi-LAT. The emission was found to correlate with massive star-forming regions and to be particularly bright towards 30 Doradus. Aims: Studies of the origin and transport of cosmic rays (CRs) in the Milky Way are frequently hampered by line-of-sight confusion and poor distance determination. The LMC offers a complementary way to address these questions by revealing whether and how the γ-ray emission is connected to specific objects, populations of objects, and structures in the galaxy. Methods: We revisited the γ-ray emission from the LMC using about 73 months of Fermi-LAT P7REP data in the 0.2-100 GeV range. We developed a complete spatial and spectral model of the LMC emission, for which we tested several approaches: a simple geometrical description, template-fitting, and a physically driven model for CR-induced interstellar emission. Results: In addition to identifying PSR J0540-6919 through its pulsations, we find two hard sources positionally coincident with plerion N 157B and supernova remnant N 132D, which were also detected at TeV energies with H.E.S.S. We detect an additional soft source that is currently unidentified. Extended emission dominates the total flux from the LMC. It consists of an extended component of about the size of the galaxy and additional emission from three to four regions with degree-scale sizes. If it is interpreted as CRs interacting with interstellar gas, the large-scale emission implies a large-scale population of ~1-100 GeV CRs with a density of ~30% of the local Galactic value. On top of that, the three to four small-scale emission regions would correspond to enhancements of the CR density by factors 2 to 6 or higher, possibly more energetic and younger populations of CRs compared to the large-scale population. An alternative explanation is that this is emission from an unresolved population of at least two dozen objects, such as pulsars and their nebulae or supernova remnants. This small-scale extended emission has a spatial distribution that does not clearly correlate with known components of the LMC, except for a possible relation to cavities and supergiant shells. Conclusions: The Fermi-LAT GeV observations allowed us to detect individual sources in the LMC. Three of the newly discovered sources are associated with rare and extreme objects. The 30 Doradus region is prominent in GeV γ-rays because PSR J0540-6919 and N 157B are strong emitters. The extended emission from the galaxy has an unexpected spatial distribution, and observations at higher energies and in radio may help to clarify its origin. FITS file of Fig. 1 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/586/A71

Planck intermediate results. XLIV. Structure of the Galactic magnetic field from dust polarization maps of the southern Galactic cap

NASA Astrophysics Data System (ADS)

Planck Collaboration; Aghanim, N.; Alves, M. I. R.; Arzoumanian, D.; Aumont, J.; Baccigalupi, C.; Ballardini, M.; Banday, A. J.; Barreiro, R. B.; Bartolo, N.; Basak, S.; Benabed, K.; Bernard, J.-P.; Bersanelli, M.; Bielewicz, P.; Bonavera, L.; Bond, J. R.; Borrill, J.; Bouchet, F. R.; Boulanger, F.; Bracco, A.; Bucher, M.; Burigana, C.; Calabrese, E.; Cardoso, J.-F.; Chiang, H. C.; Colombo, L. P. L.; Combet, C.; Comis, B.; Couchot, F.; Coulais, A.; Crill, B. P.; Curto, A.; Cuttaia, F.; Davis, R. J.; de Bernardis, P.; de Rosa, A.; de Zotti, G.; Delabrouille, J.; Delouis, J.-M.; Di Valentino, E.; Dickinson, C.; Diego, J. M.; Doré, O.; Douspis, M.; Ducout, A.; Dupac, X.; Dusini, S.; Efstathiou, G.; Elsner, F.; Enßlin, T. A.; Eriksen, H. K.; Falgarone, E.; Fantaye, Y.; Ferrière, K.; Finelli, F.; Frailis, M.; Fraisse, A. A.; Franceschi, E.; Frolov, A.; Galeotta, S.; Galli, S.; Ganga, K.; Génova-Santos, R. T.; Gerbino, M.; Ghosh, T.; González-Nuevo, J.; Górski, K. M.; Gratton, S.; Gregorio, A.; Gruppuso, A.; Gudmundsson, J. E.; Guillet, V.; Hansen, F. K.; Helou, G.; Henrot-Versillé, S.; Herranz, D.; Hivon, E.; Huang, Z.; Jaffe, A. H.; Jaffe, T. R.; Jones, W. C.; Keihänen, E.; Keskitalo, R.; Kisner, T. S.; Krachmalnicoff, N.; Kunz, M.; Kurki-Suonio, H.; Lagache, G.; Lähteenmäki, A.; Lamarre, J.-M.; Langer, M.; Lasenby, A.; Lattanzi, M.; Le Jeune, M.; Levrier, F.; Liguori, M.; Lilje, P. B.; López-Caniego, M.; Lubin, P. M.; Macías-Pérez, J. F.; Maggio, G.; Maino, D.; Mandolesi, N.; Mangilli, A.; Maris, M.; Martin, P. G.; Martínez-González, E.; Matarrese, S.; Mauri, N.; McEwen, J. D.; Melchiorri, A.; Mennella, A.; Migliaccio, M.; Miville-Deschênes, M.-A.; Molinari, D.; Moneti, A.; Montier, L.; Morgante, G.; Moss, A.; Naselsky, P.; Natoli, P.; Neveu, J.; Nørgaard-Nielsen, H. U.; Oppermann, N.; Oxborrow, C. A.; Pagano, L.; Paoletti, D.; Partridge, B.; Perdereau, O.; Perotto, L.; Pettorino, V.; Piacentini, F.; Plaszczynski, S.; Polenta, G.; Rachen, J. P.; Rebolo, R.; Reinecke, M.; Remazeilles, M.; Renzi, A.; Ristorcelli, I.; Rocha, G.; Rossetti, M.; Roudier, G.; Ruiz-Granados, B.; Salvati, L.; Sandri, M.; Savelainen, M.; Scott, D.; Sirignano, C.; Soler, J. D.; Suur-Uski, A.-S.; Tauber, J. A.; Tavagnacco, D.; Tenti, M.; Toffolatti, L.; Tomasi, M.; Tristram, M.; Trombetti, T.; Valiviita, J.; Vansyngel, F.; Van Tent, F.; Vielva, P.; Villa, F.; Wandelt, B. D.; Wehus, I. K.; Zacchei, A.; Zonca, A.

2016-12-01

Using data from the Planck satellite, we study the statistical properties of interstellar dust polarization at high Galactic latitudes around the south pole (b < -60°). Our aim is to advance the understanding of the magnetized interstellar medium (ISM), and to provide a modelling framework of the polarized dust foreground for use in cosmic microwave background (CMB) component-separation procedures. We examine the Stokes I, Q, and U maps at 353 GHz, and particularly the statistical distribution of the polarization fraction (p) and angle (ψ), in order to characterize the ordered and turbulent components of the Galactic magnetic field (GMF) in the solar neighbourhood. The Q and U maps show patterns at large angular scales, which we relate to the mean orientation of the GMF towards Galactic coordinates (l0,b0) = (70° ± 5°,24° ± 5°). The histogram of the observed p values shows a wide dispersion up to 25%. The histogram of ψ has a standard deviation of 12° about the regular pattern expected from the ordered GMF. We build a phenomenological model that connects the distributions of p and ψ to a statistical description of the turbulent component of the GMF, assuming a uniform effective polarization fraction (p0) of dust emission. To compute the Stokes parameters, we approximate the integration along the line of sight (LOS) as a sum over a set of N independent polarization layers, in each of which the turbulent component of the GMF is obtained from Gaussian realizations of a power-law power spectrum. We are able to reproduce the observed p and ψ distributions using a p0 value of 26%, a ratio of 0.9 between the strengths of the turbulent and mean components of the GMF, and a small value of N. The mean value of p (inferred from the fit of the large-scale patterns in the Stokes maps) is 12 ± 1%. We relate the polarization layers to the density structure and to the correlation length of the GMF along the LOS. We emphasize the simplicity of our model (involving only a few parameters), which can be easily computed on the celestial sphere to produce simulated maps of dust polarization. Our work is an important step towards a model that can be used to assess the accuracy of component-separation methods in present and future CMB experiments designed to search the B mode CMB polarization from primordial gravity waves.

Planck intermediate results: XLIV. Structure of the Galactic magnetic field from dust polarization maps of the southern Galactic cap

DOE PAGES

Aghanim, N.; Alves, M. I. R.; Arzoumanian, D.; ...

2016-12-12

Using data from the Planck satellite, we study in this paper the statistical properties of interstellar dust polarization at high Galactic latitudes around the south pole (b < -60°). Our aim is to advance the understanding of the magnetized interstellar medium (ISM), and to provide a modelling framework of the polarized dust foreground for use in cosmic microwave background (CMB) component-separation procedures. We examine the Stokes I, Q, and U maps at 353 GHz, and particularly the statistical distribution of the polarization fraction (p) and angle (ψ), in order to characterize the ordered and turbulent components of the Galactic magneticmore » field (GMF) in the solar neighbourhood. The Q and U maps show patterns at large angular scales, which we relate to the mean orientation of the GMF towards Galactic coordinates (l 0,b 0) = (70° ± 5°,24° ± 5°). The histogram of the observed p values shows a wide dispersion up to 25%. The histogram of ψ has a standard deviation of 12° about the regular pattern expected from the ordered GMF. We build a phenomenological model that connects the distributions of p and ψ to a statistical description of the turbulent component of the GMF, assuming a uniform effective polarization fraction (p 0) of dust emission. To compute the Stokes parameters, we approximate the integration along the line of sight (LOS) as a sum over a set of N independent polarization layers, in each of which the turbulent component of the GMF is obtained from Gaussian realizations of a power-law power spectrum. We are able to reproduce the observed p and ψ distributions using a p 0 value of 26%, a ratio of 0.9 between the strengths of the turbulent and mean components of the GMF, and a small value of N. The mean value of p (inferred from the fit of the large-scale patterns in the Stokes maps) is 12 ± 1%. We relate the polarization layers to the density structure and to the correlation length of the GMF along the LOS. We emphasize the simplicity of our model (involving only a few parameters), which can be easily computed on the celestial sphere to produce simulated maps of dust polarization. Finally, our work is an important step towards a model that can be used to assess the accuracy of component-separation methods in present and future CMB experiments designed to search the B mode CMB polarization from primordial gravity waves.« less

Planck intermediate results: XLIV. Structure of the Galactic magnetic field from dust polarization maps of the southern Galactic cap

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aghanim, N.; Alves, M. I. R.; Arzoumanian, D.

Using data from the Planck satellite, we study in this paper the statistical properties of interstellar dust polarization at high Galactic latitudes around the south pole (b < -60°). Our aim is to advance the understanding of the magnetized interstellar medium (ISM), and to provide a modelling framework of the polarized dust foreground for use in cosmic microwave background (CMB) component-separation procedures. We examine the Stokes I, Q, and U maps at 353 GHz, and particularly the statistical distribution of the polarization fraction (p) and angle (ψ), in order to characterize the ordered and turbulent components of the Galactic magneticmore » field (GMF) in the solar neighbourhood. The Q and U maps show patterns at large angular scales, which we relate to the mean orientation of the GMF towards Galactic coordinates (l 0,b 0) = (70° ± 5°,24° ± 5°). The histogram of the observed p values shows a wide dispersion up to 25%. The histogram of ψ has a standard deviation of 12° about the regular pattern expected from the ordered GMF. We build a phenomenological model that connects the distributions of p and ψ to a statistical description of the turbulent component of the GMF, assuming a uniform effective polarization fraction (p 0) of dust emission. To compute the Stokes parameters, we approximate the integration along the line of sight (LOS) as a sum over a set of N independent polarization layers, in each of which the turbulent component of the GMF is obtained from Gaussian realizations of a power-law power spectrum. We are able to reproduce the observed p and ψ distributions using a p 0 value of 26%, a ratio of 0.9 between the strengths of the turbulent and mean components of the GMF, and a small value of N. The mean value of p (inferred from the fit of the large-scale patterns in the Stokes maps) is 12 ± 1%. We relate the polarization layers to the density structure and to the correlation length of the GMF along the LOS. We emphasize the simplicity of our model (involving only a few parameters), which can be easily computed on the celestial sphere to produce simulated maps of dust polarization. Finally, our work is an important step towards a model that can be used to assess the accuracy of component-separation methods in present and future CMB experiments designed to search the B mode CMB polarization from primordial gravity waves.« less

Quantum Chemistry Meets Spectroscopy for Astrochemistry: Increasing Complexity toward Prebiotic Molecules.

PubMed

Barone, Vincenzo; Biczysko, Malgorzata; Puzzarini, Cristina

2015-05-19

For many years, scientists suspected that the interstellar medium was too hostile for organic species and that only a few simple molecules could be formed under such extreme conditions. However, the detection of approximately 180 molecules in interstellar or circumstellar environments in recent decades has changed this view dramatically. A rich chemistry has emerged, and relatively complex molecules such as C60 and C70 are formed. Recently, researchers have also detected complex organic and potentially prebiotic molecules, such as amino acids, in meteorites and in other space environments. Those discoveries have further stimulated the debate on the origin of the building blocks of life in the universe. Many efforts continue to focus on the physical, chemical, and astrophysical processes by which prebiotic molecules can be formed in the interstellar dust and dispersed to Earth or to other planets.Spectroscopic techniques, which are widely used to infer information about molecular structure and dynamics, play a crucial role in the investigation of planetary atmosphere and the interstellar medium. Increasingly these astrochemical investigations are assisted by quantum-mechanical calculations of structures as well as spectroscopic and thermodynamic properties, such as transition frequencies and reaction enthalpies, to guide and support observations, line assignments, and data analysis in these new and chemically complicated situations. However, it has proved challenging to extend accurate quantum-chemical computational approaches to larger systems because of the unfavorable scaling with the number of degrees of freedom (both electronic and nuclear).In this Account, we show that it is now possible to compute physicochemical properties of building blocks of biomolecules with an accuracy rivaling that of the most sophisticated experimental techniques, and we summarize specific contributions from our groups. As a test case, we present the underlying computational machinery through the investigation of oxirane. We describe how we determine the molecular structure and then how we characterize the rotational and IR spectra, the most important issues for a correct theoretical description and a proper comparison with experiment. Next, we analyze the spectroscopic properties of representative building blocks of DNA bases (uracil and pyrimidine) and of proteins (glycine and glycine dipeptide analogue).Solvation, surface chemistry (dust fraction, adsorption, desorption), and inter- and intramolecular interactions, such as self-organization and self-interaction, are important molecular processes for understanding astrochemistry. Using the specific cases of uracil dimers and glycine adsorbed on silicon grains, we also illustrate approaches in which we treat different regions, interactions, or effects at different levels of sophistication.

Modeling Shocks Detected by Voyager 1 in the Local Interstellar Medium

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, T. K.; Pogorelov, N. V.; Burlaga, L. F.

The magnetometer (MAG) on Voyager 1 ( V1 ) has been sampling the interstellar magnetic field (ISMF) since 2012 August. The V1 MAG observations have shown draped ISMF in the very local interstellar medium disturbed occasionally by significant enhancements in magnetic field strength. Using a three-dimensional, data-driven, multi-fluid model, we investigated these magnetic field enhancements beyond the heliopause that are supposedly associated with solar transients. To introduce time-dependent effects at the inner boundary at 1 au, we used daily averages of the solar wind parameters from the OMNI data set. The model ISMF strength, direction, and proton number density aremore » compared with V1 data beyond the heliopause. The model reproduced the large-scale fluctuations between 2012.652 and 2016.652, including major events around 2012.9 and 2014.6. The model also predicts shocks arriving at V1 around 2017.395 and 2019.502. Another model driven by OMNI data with interplanetary coronal mass ejections (ICMEs) removed at the inner boundary suggests that ICMEs may play a significant role in the propagation of shocks into the interstellar medium.« less

Reconciling Dwarf Galaxies with ΛCDM Cosmology: Simulating a Realistic Population of Satellites around a Milky Way-mass Galaxy

NASA Astrophysics Data System (ADS)

Wetzel, Andrew R.; Hopkins, Philip F.; Kim, Ji-hoon; Faucher-Giguère, Claude-André; Kereš, Dušan; Quataert, Eliot

2016-08-01

Low-mass “dwarf” galaxies represent the most significant challenges to the cold dark matter (CDM) model of cosmological structure formation. Because these faint galaxies are (best) observed within the Local Group (LG) of the Milky Way (MW) and Andromeda (M31), understanding their formation in such an environment is critical. We present first results from the Latte Project: the Milky Way on Feedback in Realistic Environments (FIRE). This simulation models the formation of an MW-mass galaxy to z=0 within ΛCDM cosmology, including dark matter, gas, and stars at unprecedented resolution: baryon particle mass of 7070 {M}⊙ with gas kernel/softening that adapts down to 1 {pc} (with a median of 25{--}60 {pc} at z=0). Latte was simulated using the GIZMO code with a mesh-free method for accurate hydrodynamics and the FIRE-2 model for star formation and explicit feedback within a multi-phase interstellar medium. For the first time, Latte self-consistently resolves the spatial scales corresponding to half-light radii of dwarf galaxies that form around an MW-mass host down to {M}{star}≳ {10}5 {M}⊙ . Latte’s population of dwarf galaxies agrees with the LG across a broad range of properties: (1) distributions of stellar masses and stellar velocity dispersions (dynamical masses), including their joint relation; (2) the mass-metallicity relation; and (3) diverse range of star formation histories, including their mass dependence. Thus, Latte produces a realistic population of dwarf galaxies at {M}{star}≳ {10}5 {M}⊙ that does not suffer from the “missing satellites” or “too big to fail” problems of small-scale structure formation. We conclude that baryonic physics can reconcile observed dwarf galaxies with standard ΛCDM cosmology.

Spectroscopy of prospective interstellar ions and radicals isolated in para-hydrogen matrices.

PubMed

Tsuge, Masashi; Tseng, Chih-Yu; Lee, Yuan-Pern

2018-02-21

para-Hydrogen (p-H 2 ) serves as a new host in matrix-isolation experiments for an investigation of species of astrochemical interest. Protonated and mono-hydrogenated species are produced upon electron bombardment during deposition of p-H 2 containing a precursor in a small proportion. The applications of this novel technique to generate protonated polycyclic aromatic hydrocarbons (H + PAH), protonated polycyclic nitrogen heterocycles (H + PANH), and their neutral counterparts, which are important in the identification of interstellar unidentified infrared emission bands, demonstrate its superiority over other methods. The clean production with little fragmentation, ease of distinction between protonated and neutral species, narrow lines and reliable relative infrared intensities of the lines, and broad coverage of the spectral range associated with this method enable us to assign the isomers unambiguously. The application of this method to the protonation of small molecules is more complicated partly because of the feasible fragmentation and reactions, and partly because of the possible proton sharing between the species of interest and H 2 , but, with isotopic experiments and secondary photolysis, definitive assignments are practicable. Furthermore, the true relative infrared intensities are critical to a comparison of experimental results with data from theoretical calculations. The spectra of a proton-shared species in solid p-H 2 might provide insight into a search for spectra of proton-bound species in interstellar media. Investigations of hydrogenated species involving the photolysis of Cl 2 or precursors of OH complement those using electron bombardment and provide an improved ratio of signal to noise. With careful grouping of observed lines after secondary photolysis and a comparison with theoretical predictions, various isomers of these species have been determined. This photolytic technique has been applied in an investigation of hydrogenated PAH and PANH, and the hydrogenation reactions of small molecules, which are important in interstellar ice and the evolution of life. The electronic transitions of molecules in solid p-H 2 have been little investigated. The matrix shift of the origins of transitions and the spectral width seem to be much smaller than those of noble-gas matrices; these features might facilitate a direct comparison of matrix spectra with diffuse interstellar bands, but further data are required to assess this possibility. The advantages and disadvantages of applying these techniques of p-H 2 matrix isolation to astrochemical research and their future perspectives are discussed.

Dust evolution, a global view: III. Core/mantle grains, organic nano-globules, comets and surface chemistry

NASA Astrophysics Data System (ADS)

Jones, A. P.

2016-12-01

Within the framework of The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS), this work explores the surface processes and chemistry relating to core/mantle interstellar and cometary grain structures and their influence on the nature of these fascinating particles. It appears that a realistic consideration of the nature and chemical reactivity of interstellar grain surfaces could self-consistently and within a coherent framework explain: the anomalous oxygen depletion, the nature of the CO dark gas, the formation of `polar ice' mantles, the red wing on the 3 μm water ice band, the basis for the O-rich chemistry observed in hot cores, the origin of organic nano-globules and the 3.2 μm `carbonyl' absorption band observed in comet reflectance spectra. It is proposed that the reaction of gas phase species with carbonaceous a-C(:H) grain surfaces in the interstellar medium, in particular the incorporation of atomic oxygen into grain surfaces in epoxide functional groups, is the key to explaining these observations. Thus, the chemistry of cosmic dust is much more intimately related with that of the interstellar gas than has previously been considered. The current models for interstellar gas and dust chemistry will therefore most likely need to be fundamentally modified to include these new grain surface processes.

Magnetic seismology of interstellar gas clouds: Unveiling a hidden dimension.

PubMed

Tritsis, Aris; Tassis, Konstantinos

2018-05-11

Stars and planets are formed inside dense interstellar molecular clouds by processes imprinted on the three-dimensional (3D) morphology of the clouds. Determining the 3D structure of interstellar clouds remains challenging because of projection effects and difficulties measuring the extent of the clouds along the line of sight. We report the detection of normal vibrational modes in the isolated interstellar cloud Musca, allowing determination of the 3D physical dimensions of the cloud. We found that Musca is vibrating globally, with the characteristic modes of a sheet viewed edge on, not the characteristics of a filament as previously supposed. We reconstructed the physical properties of Musca through 3D magnetohydrodynamic simulations, reproducing the observed normal modes and confirming a sheetlike morphology. Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.

Extinct radioactivities - A three-phase mixing model. [for early solar system abundances

NASA Technical Reports Server (NTRS)

Clayton, D. D.

1983-01-01

A new class of models is advanced for interpreting the relationship of radioactive abundances in the early solar system to their average concentration in the interstellar medium. The model assumes that fresh radioactivities are ejected from supernovae into the hot interstellar medium, and that the time scales for changes of phase into molecular clouds determine how much survives for formation therein of the solar system. A more realistic and physically motivated understanding of the low observed concentrations of I-129, Pu-244, and Pd-107 may result.

Planck intermediate results: XXXII. The relative orientation between the magnetic field and structures traced by interstellar dust

DOE PAGES

Adam, R.; Ade, P. A. R.; Aghanim, N.; ...

2016-02-09

The role of the magnetic field in the formation of the filamentary structures observed in the interstellar medium (ISM) is a debated topic owing to the paucity of relevant observations needed to test existing models. The Planck all-sky maps of linearly polarized emission from dust at 353 GHz provide the required combination of imaging and statistics to study the correlation between the structures of the Galactic magnetic field and of interstellar matter over the whole sky, both in the diffuse ISM and in molecular clouds. The data reveal that structures, or ridges, in the intensity map have counterparts in themore » Stokes Q and/or U maps. In this paper, we focus our study on structures at intermediate and high Galactic latitudes, which cover two orders of magnitude in column density, from 10 20 to 10 22 cm -2. We measure the magnetic field orientation on the plane ofthe sky from the polarization data, and present an algorithm to estimate the orientation of the ridges from the dust intensity map. We use analytical models to account for projection effects. Comparing polarization angles on and off the structures, we estimate the mean ratio between the strengths of the turbulent and mean components of the magnetic field to be between 0.6 and 1.0, with a preferred value of 0.8. We find that the ridges are usually aligned with the magnetic field measured on the structures. This statistical trend becomes more striking for increasing polarization fraction and decreasing column density. There is no alignment for the highest column density ridges. We interpret the increase in alignment with polarization fraction as a consequence of projection effects. We present maps to show that the decrease in alignment for high column density is not due to a loss of correlation between the distribution of matter and the geometry of the magnetic field. In molecular complexes, we also observe structures perpendicular to the magnetic field, which, statistically, cannot be accounted for by projection effects. This first statistical study of the relative orientation between the matter structures and the magnetic field in the ISM points out that, at the angular scales probed by Planck, the field geometry projected on the plane of the sky is correlated with the distribution of matter. In the diffuse ISM, the structures of matter are usually aligned with the magnetic field, while perpendicular structures appear in molecular clouds. Finally, we discuss our results in the context of models and MHD simulations, which attempt to describe the respective roles of turbulence, magnetic field, and self-gravity in the formation of structures in the magnetized ISM.« less

Planck intermediate results. XXXII. The relative orientation between the magnetic field and structures traced by interstellar dust

NASA Astrophysics Data System (ADS)

Planck Collaboration; Adam, R.; Ade, P. A. R.; Aghanim, N.; Alves, M. I. R.; Arnaud, M.; Arzoumanian, D.; Ashdown, M.; Aumont, J.; Baccigalupi, C.; Banday, A. J.; Barreiro, R. B.; Bartolo, N.; Battaner, E.; Benabed, K.; Benoit-Lévy, A.; Bernard, J.-P.; Bersanelli, M.; Bielewicz, P.; Bonaldi, A.; Bonavera, L.; Bond, J. R.; Borrill, J.; Bouchet, F. R.; Boulanger, F.; Bracco, A.; Burigana, C.; Butler, R. C.; Calabrese, E.; Cardoso, J.-F.; Catalano, A.; Chamballu, A.; Chiang, H. C.; Christensen, P. R.; Colombi, S.; Colombo, L. P. L.; Combet, C.; Couchot, F.; Crill, B. P.; Curto, A.; Cuttaia, F.; Danese, L.; Davies, R. D.; Davis, R. J.; de Bernardis, P.; de Rosa, A.; de Zotti, G.; Delabrouille, J.; Dickinson, C.; Diego, J. M.; Dole, H.; Donzelli, S.; Doré, O.; Douspis, M.; Ducout, A.; Dupac, X.; Efstathiou, G.; Elsner, F.; Enßlin, T. A.; Eriksen, H. K.; Falgarone, E.; Ferrière, K.; Finelli, F.; Forni, O.; Frailis, M.; Fraisse, A. A.; Franceschi, E.; Frejsel, A.; Galeotta, S.; Galli, S.; Ganga, K.; Ghosh, T.; Giard, M.; Gjerløw, E.; González-Nuevo, J.; Górski, K. M.; Gregorio, A.; Gruppuso, A.; Guillet, V.; Hansen, F. K.; Hanson, D.; Harrison, D. L.; Henrot-Versillé, S.; Hernández-Monteagudo, C.; Herranz, D.; Hildebrandt, S. R.; Hivon, E.; Hobson, M.; Holmes, W. A.; Hovest, W.; Huffenberger, K. M.; Hurier, G.; Jaffe, A. H.; Jaffe, T. R.; Jones, W. C.; Juvela, M.; Keihänen, E.; Keskitalo, R.; Kisner, T. S.; Kneissl, R.; Knoche, J.; Kunz, M.; Kurki-Suonio, H.; Lagache, G.; Lamarre, J.-M.; Lasenby, A.; Lattanzi, M.; Lawrence, C. R.; Leonardi, R.; Levrier, F.; Liguori, M.; Lilje, P. B.; Linden-Vørnle, M.; López-Caniego, M.; Lubin, P. M.; Macías-Pérez, J. F.; Maffei, B.; Maino, D.; Mandolesi, N.; Maris, M.; Marshall, D. J.; Martin, P. G.; Martínez-González, E.; Masi, S.; Matarrese, S.; Mazzotta, P.; Melchiorri, A.; Mendes, L.; Mennella, A.; Migliaccio, M.; Miville-Deschênes, M.-A.; Moneti, A.; Montier, L.; Morgante, G.; Mortlock, D.; Munshi, D.; Murphy, J. A.; Naselsky, P.; Natoli, P.; Nørgaard-Nielsen, H. U.; Noviello, F.; Novikov, D.; Novikov, I.; Oppermann, N.; Oxborrow, C. A.; Pagano, L.; Pajot, F.; Paoletti, D.; Pasian, F.; Perdereau, O.; Perotto, L.; Perrotta, F.; Pettorino, V.; Piacentini, F.; Piat, M.; Plaszczynski, S.; Pointecouteau, E.; Polenta, G.; Ponthieu, N.; Popa, L.; Pratt, G. W.; Prunet, S.; Puget, J.-L.; Rachen, J. P.; Reach, W. T.; Reinecke, M.; Remazeilles, M.; Renault, C.; Ristorcelli, I.; Rocha, G.; Roudier, G.; Rubiño-Martín, J. A.; Rusholme, B.; Sandri, M.; Santos, D.; Savini, G.; Scott, D.; Soler, J. D.; Spencer, L. D.; Stolyarov, V.; Sudiwala, R.; Sunyaev, R.; Sutton, D.; Suur-Uski, A.-S.; Sygnet, J.-F.; Tauber, J. A.; Terenzi, L.; Toffolatti, L.; Tomasi, M.; Tristram, M.; Tucci, M.; Umana, G.; Valenziano, L.; Valiviita, J.; Van Tent, B.; Vielva, P.; Villa, F.; Wade, L. A.; Wandelt, B. D.; Wehus, I. K.; Wiesemeyer, H.; Yvon, D.; Zacchei, A.; Zonca, A.

2016-02-01

The role of the magnetic field in the formation of the filamentary structures observed in the interstellar medium (ISM) is a debated topic owing to the paucity of relevant observations needed to test existing models. The Planck all-sky maps of linearly polarized emission from dust at 353 GHz provide the required combination of imaging and statistics to study the correlation between the structures of the Galactic magnetic field and of interstellar matter over the whole sky, both in the diffuse ISM and in molecular clouds. The data reveal that structures, or ridges, in the intensity map have counterparts in the Stokes Q and/or U maps. We focus our study on structures at intermediate and high Galactic latitudes, which cover two orders of magnitude in column density, from 1020 to 1022 cm-2. We measure the magnetic field orientation on the plane ofthe sky from the polarization data, and present an algorithm to estimate the orientation of the ridges from the dust intensity map. We use analytical models to account for projection effects. Comparing polarization angles on and off the structures, we estimate the mean ratio between the strengths of the turbulent and mean components of the magnetic field to be between 0.6 and 1.0, with a preferred value of 0.8. We find that the ridges are usually aligned with the magnetic field measured on the structures. This statistical trend becomes more striking for increasing polarization fraction and decreasing column density. There is no alignment for the highest column density ridges. We interpret the increase in alignment with polarization fraction as a consequence of projection effects. We present maps to show that the decrease in alignment for high column density is not due to a loss of correlation between the distribution of matter and the geometry of the magnetic field. In molecular complexes, we also observe structures perpendicular to the magnetic field, which, statistically, cannot be accounted for by projection effects. This first statistical study of the relative orientation between the matter structures and the magnetic field in the ISM points out that, at the angular scales probed by Planck, the field geometry projected on the plane of the sky is correlated with the distribution of matter. In the diffuse ISM, the structures of matter are usually aligned with the magnetic field, while perpendicular structures appear in molecular clouds. We discuss our results in the context of models and MHD simulations, which attempt to describe the respective roles of turbulence, magnetic field, and self-gravity in the formation of structures in the magnetized ISM.

Planck intermediate results: XXXII. The relative orientation between the magnetic field and structures traced by interstellar dust

DOE Office of Scientific and Technical Information (OSTI.GOV)

Adam, R.; Ade, P. A. R.; Aghanim, N.

The role of the magnetic field in the formation of the filamentary structures observed in the interstellar medium (ISM) is a debated topic owing to the paucity of relevant observations needed to test existing models. The Planck all-sky maps of linearly polarized emission from dust at 353 GHz provide the required combination of imaging and statistics to study the correlation between the structures of the Galactic magnetic field and of interstellar matter over the whole sky, both in the diffuse ISM and in molecular clouds. The data reveal that structures, or ridges, in the intensity map have counterparts in themore » Stokes Q and/or U maps. In this paper, we focus our study on structures at intermediate and high Galactic latitudes, which cover two orders of magnitude in column density, from 10 20 to 10 22 cm -2. We measure the magnetic field orientation on the plane ofthe sky from the polarization data, and present an algorithm to estimate the orientation of the ridges from the dust intensity map. We use analytical models to account for projection effects. Comparing polarization angles on and off the structures, we estimate the mean ratio between the strengths of the turbulent and mean components of the magnetic field to be between 0.6 and 1.0, with a preferred value of 0.8. We find that the ridges are usually aligned with the magnetic field measured on the structures. This statistical trend becomes more striking for increasing polarization fraction and decreasing column density. There is no alignment for the highest column density ridges. We interpret the increase in alignment with polarization fraction as a consequence of projection effects. We present maps to show that the decrease in alignment for high column density is not due to a loss of correlation between the distribution of matter and the geometry of the magnetic field. In molecular complexes, we also observe structures perpendicular to the magnetic field, which, statistically, cannot be accounted for by projection effects. This first statistical study of the relative orientation between the matter structures and the magnetic field in the ISM points out that, at the angular scales probed by Planck, the field geometry projected on the plane of the sky is correlated with the distribution of matter. In the diffuse ISM, the structures of matter are usually aligned with the magnetic field, while perpendicular structures appear in molecular clouds. Finally, we discuss our results in the context of models and MHD simulations, which attempt to describe the respective roles of turbulence, magnetic field, and self-gravity in the formation of structures in the magnetized ISM.« less

The Phenomenology of Small-Scale Turbulence

NASA Astrophysics Data System (ADS)

Sreenivasan, K. R.; Antonia, R. A.

I have sometimes thought that what makes a man's work classic is often just this multiplicity [of interpretations], which invites and at the same time resists our craving for a clear understanding. Wright (1982, p. 34), on Wittgenstein's philosophy Small-scale turbulence has been an area of especially active research in the recent past, and several useful research directions have been pursued. Here, we selectively review this work. The emphasis is on scaling phenomenology and kinematics of small-scale structure. After providing a brief introduction to the classical notions of universality due to Kolmogorov and others, we survey the existing work on intermittency, refined similarity hypotheses, anomalous scaling exponents, derivative statistics, intermittency models, and the structure and kinematics of small-scale structure - the latter aspect coming largely from the direct numerical simulation of homogeneous turbulence in a periodic box.

Faraday rotation measure variations in the Cygnus region and the spectrum of interstellar plasma turbulence

NASA Technical Reports Server (NTRS)

Lazio, T. Joseph; Spangler, Steven R.; Cordes, James M.

1990-01-01

Linear polarization observations were made of eight double-lobed radio galaxies viewed through the galactic plane in the Cygnus region. These observations have been used to determine intra- and intersource rotation measure differences; in some cases, unambiguous rotation measures have been extracted. The rotation measures are dominated by foreground magnetoionic material. The differences in rotation measure between pairs of sources correlate with angular separation for separations from 10 arcsec to 1.5 deg. These rotation measure fluctuations are consistent with a model in which the electron density varies on roughly 0.1-200 pc scales. The amplitudes of these variations are, in turn, consistent with those electron density variations that cause diffractive interstellar scattering on scales less than 10 to the 11th cm.

Metastability of isoformyl ions in collisions with helium and hydrogen. [in interstellar molecular clouds

NASA Technical Reports Server (NTRS)

Green, S.

1984-01-01

The stability of HOC(+) ions under conditions in interstellar molecular clouds is considered. In particular, the possibility that collisions with helium or hydrogen will induce isomerization to the stable HCO(+) form is examined theoretically. Portions of the electronic potential energy surfaces for interaction with He and H atoms are obtained from standard quantum mechanical calculations. Collisions with He atoms are found to be totally ineffective for inducing isomerization. Collisions with H atoms are found to be ineffective at low interstellar temperatures owing to a small (about 500 K) barrier in the entrance channel; at higher temperatures where this barrier can be overcome, however, collisions with hydrogen atoms do result in conversion to the stable HCO(+) form. Although detailed calculations are not presented, it is argued that low-energy collisions with H2 molecules are also ineffective in destroying the metastable ion.

The rate of the reaction between CN and C2H2 at interstellar temperatures.

PubMed

Woon, D E; Herbst, E

1997-03-01

The rate coefficient for the important interstellar reaction between CN and C2H2 has been calculated as a function of temperature between 10 and 300 K. The potential surface for this reaction has been determined through ab initio quantum chemical techniques; the potential exhibits no barrier in the entrance channel but does show a small exit channel barrier, which lies below the energy of reactants. Phase-space calculations for the reaction dynamics, which take the exit channel barrier into account, show the same unusual temperature dependence as determined by experiment, in which the rate coefficient at first increases as the temperature is reduced below room temperature and then starts to decrease as the temperature drops below 50-100 K. The agreement between theory and experiment provides strong confirmation that the reaction occurs appreciably at cool interstellar temperatures.

Interstellar Travel without 'Magic'

NASA Astrophysics Data System (ADS)

Woodcock, G.

The possibility of interstellar space travel has become a popular subject. Distances of light years are an entirely new realm for human space travel. New means of propulsion are needed. Speculation about propulsion has included "magic", space warps, faster-than-light travel, known physics such as antimatter for which no practical implementation is known and also physics for which current research offers at least a hint of implementation, i.e. fusion. Performance estimates are presented for the latter and used to create vehicle concepts. Fusion propulsion will mean travel times of hundreds of years, so we adopt the "space colony" concepts of O'Neill as a ship design that could support a small civilization indefinitely; this provides the technical means. Economic reasoning is presented, arguing that development and production of "space colony" habitats for relief of Earth's population, with addition of fusion engines, will lead to vessels that can go interstellar. Scenarios are presented and a speculative estimate of a timetable is given.

Morphological diagnostics of star formation in molecular clouds

NASA Astrophysics Data System (ADS)

Beaumont, Christopher Norris

Molecular clouds are the birth sites of all star formation in the present-day universe. They represent the initial conditions of star formation, and are the primary medium by which stars transfer energy and momentum back to parsec scales. Yet, the physical evolution of molecular clouds remains poorly understood. This is not due to a lack of observational data, nor is it due to an inability to simulate the conditions inside molecular clouds. Instead, the physics and structure of the interstellar medium are sufficiently complex that interpreting molecular cloud data is very difficult. This dissertation mitigates this problem, by developing more sophisticated ways to interpret morphological information in molecular cloud observations and simulations. In particular, I have focused on leveraging machine learning techniques to identify physically meaningful substructures in the interstellar medium, as well as techniques to inter-compare molecular cloud simulations to observations. These contributions make it easier to understand the interplay between molecular clouds and star formation. Specific contributions include: new insight about the sheet-like geometry of molecular clouds based on observations of stellar bubbles; a new algorithm to disambiguate overlapping yet morphologically distinct cloud structures; a new perspective on the relationship between molecular cloud column density distributions and the sizes of cloud substructures; a quantitative analysis of how projection effects affect measurements of cloud properties; and an automatically generated, statistically-calibrated catalog of bubbles identified from their infrared morphologies.

Interstellar X-Ray Absorption Spectroscopy of the Crab Pulsar with the LETGS

NASA Technical Reports Server (NTRS)

Paerels, Frits; Weisskopf, Martin C.; Tennant, Allyn F.; ODell, Stephen L.; Swartz, Douglas A.; Kahn, Steven M.; Behar, Ehud; Becker, Werner; Whitaker, Ann F. (Technical Monitor)

2001-01-01

We study the interstellar X-ray absorption along the line of sight to the Crab Pulsar. The Crab was observed with the Low Energy Transmission Grating Spectrometer on the Chandra X-ray Observatory, and the pulsar, a point source, produces a full resolution spectrum. The continuum spectrum appears smooth, and we compare its parameters with other measurements of the pulsar spectrum. The spectrum clearly shows absorption edges due to interstellar Ne, Fe, and O. The O edge shows spectral structure that is probably due to O bound in molecules or dust. We search for near-edge structure (EXAFS) in the O absorption spectrum. The Fe L absorption spectrum is largely due to a set of unresolved discrete n=2-3 transitions in neutral or near-neutral Fe, and we analyze it using a new set of dedicated atomic structure calculations, which provide absolute cross sections. In addition to being interesting in its own right, the ISM absorption needs to be understood in quantitative detail in order to derive spectroscopic constraints on possible soft thermal radiation from the pulsar.

A milestone toward understanding PDR properties in the extreme environment of LMC-30 Doradus

NASA Astrophysics Data System (ADS)

Chevance, M.; Madden, S. C.; Lebouteiller, V.; Godard, B.; Cormier, D.; Galliano, F.; Hony, S.; Indebetouw, R.; Le Bourlot, J.; Lee, M.-Y.; Le Petit, F.; Pellegrini, E.; Roueff, E.; Wu, R.

2016-05-01

Context. More complete knowledge of galaxy evolution requires understanding the process of star formation and the interaction between the interstellar radiation field and interstellar medium (ISM) in galactic environments traversing a wide range of physical parameter space. We focus on the impact of massive star formation on the surrounding low metallicity ISM in 30 Doradus in the Large Magellanic Cloud (LMC). A low metal abundance, which can characterizes some galaxies of the early Universe, results in less ultraviolet (UV) shielding for the formation of the molecular gas necessary for star formation to proceed. The half-solar metallicity gas in this region is strongly irradiated by the super star cluster R136, making it an ideal laboratory to study the structure of the ISM in an extreme environment. Aims: Our goal is to construct a comprehensive, self-consistent picture of the density, radiation field, and ISM structure in the most active star-forming region in the LMC, 30 Doradus. Our spatially resolved study investigates the gas heating and cooling mechanisms, particularly in the photodissociation regions (PDR) where the chemistry and thermal balance are regulated by far-UV photons (6 eV < hν < 13.6 eV). Methods: We present Herschel observations of far-infrared (FIR) fine-structure lines obtained with PACS and SPIRE/FTS. We combined atomic fine-structure lines from Herschel and Spitzer observations with ground-based CO data to provide diagnostics on the properties and structure of the gas by modeling it with the Meudon PDR code. For each tracer we estimate the possible contamination from the ionized gas to isolate the PDR component. We derive the spatial distribution of the radiation field, the pressure, the size, and the filling factor of the photodissociated gas and molecular clouds. Results: We find a range of pressure of ~105-1.7 × 106 cm-3 K and a range of incident radiation field GUV~102-2.5 × 104 through PDR modeling. Assuming a plane-parallel geometry and a uniform medium, we find a total extinction AVmax of 1-3 mag, which corresponds to a PDR cloud size of 0.2 to 3pc with small CO depth scale of 0.06 to 0.5 pc. At least 90% of the [C II] originates in PDRs in this region, while a significant fraction of the LFIR (up to 70% in some places) can be associated with an ionized gas component. The high [O III]/[C II] ratio (2 to 60) throughout the observed map, correlated with the filling factor, reveals the porosity of the ISM in this region, which is traversed by hard UV photons surrounding small PDR clumps. We also determine the three-dimensional structure of the gas, showing that the clouds are distributed 20 to 80 pc away from the main ionizing cluster, R136. The reduced images are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/590/A36

Modeling the Oxygen K Absorption in the Interstellar Medium: An XMM-Newton View of Sco X-1

NASA Technical Reports Server (NTRS)

Garcia, J.; Ramirez, J. M.; Kallman, T. R.; Witthoeft, M.; Bautista, M. A.; Mendoza, C.; Palmeri, P.; Quinet, P.

2011-01-01

We investigate the absorption structure of the oxygen in the interstellar medium by analyzing XMM-Newton observations of the low mass X-ray binary Sco X-1. We use simple models based on the O I atomic cross section from different sources to fit the data and evaluate the impact of the atomic data in the interpretation of astrophysical observations. We show that relatively small differences in the atomic calculations can yield spurious results. We also show that the most complete and accurate set of atomic cross sections successfully reproduce the observed data in the 21 - 24.5 Angstrom wavelength region of the spectrum. Our fits indicate that the absorption is mainly due to neutral gas with an ionization parameter of Epsilon = 10(exp -4) erg/sq cm, and an oxygen column density of N(sub O) approx. = 8-10 x 10(exp 17)/sq cm. Our models are able to reproduce both the K edge and the K(alpha) absorption line from O I, which are the two main features in this region. We find no conclusive evidence for absorption by other than atomic oxygen.

H i and CO in the circumstellar environment of the S-type star RS Cancri

NASA Astrophysics Data System (ADS)

Libert, Y.; Winters, J. M.; Le Bertre, T.; Gérard, E.; Matthews, L. D.

2010-06-01

Context. The history of mass loss during the AGB phase is key to understanding the stellar evolution and the gas and dust replenishment of the interstellar medium. The mass-loss phenomenon presents fluctuations with a wide variety of timescales and spatial scales and requires combining data from multiple tracers. Aims: We study the respective contributions of the central source and of the external medium to the complex geometry of circumstellar ejecta. Methods: This paper presents Plateau de Bure Interferometer and IRAM 30-m telescope CO rotational line observations, along with H i data obtained with the Nançay Radio Telescope for the oxygen-rich semi-regular variable RS Cnc, in order to probe its circumstellar environment on different scales. Results: We detect both the CO(1-0) and the CO(2-1) rotational lines from RS Cnc. The line profiles are composite, comprising two components of half-width ~2 km s-1 and ~8 km s-1, respectively. Whereas the narrow velocity component seems to originate in an equatorial disk in the central part of the CO envelope, the broad component reveals a bipolar structure, with a north-south velocity gradient. In addition, we obtain new H i data on the source and around it in a field of almost 1 square degree. The H i line is centered on vLSR = 7 km s-1 in agreement with CO observations. A new reduction process reveals a complex extended structure in the northwest direction, of estimated size ~18', with a position angle (~310°) opposite the direction of the stellar proper motion (~140°). We derive an H i mass of ~3 × 10-2 M_⊙ for this structure. Based on a non spherical simulation, we find that this structure is consistent with arising from the interaction of the star undergoing mass loss at an average rate of ~10-7 M⊙ yr-1 over ~2-3 × 105 years with the interstellar medium. Conclusions: Using CO and H i lines, we show that the circumstellar environment around RS Cnc includes two related but well separated regions. With CO, we find a bipolar geometry that probably originates from the intrinsic behavior of recent mass-loss processes. With H i, we find a trail of gas, in a direction opposite to the proper motion of RS Cnc lending support to the hypothesis of an interaction with the interstellar medium. This work illustrates the powerful complementarity of CO and H i observations with regard to a more complete description of circumstellar environments around AGB stars. Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).

The enigmatic nature of the circumstellar envelope and bow shock surrounding Betelgeuse as revealed by Herschel. I. Evidence of clumps, multiple arcs, and a linear bar-like structure

NASA Astrophysics Data System (ADS)

Decin, L.; Cox, N. L. J.; Royer, P.; Van Marle, A. J.; Vandenbussche, B.; Ladjal, D.; Kerschbaum, F.; Ottensamer, R.; Barlow, M. J.; Blommaert, J. A. D. L.; Gomez, H. L.; Groenewegen, M. A. T.; Lim, T.; Swinyard, B. M.; Waelkens, C.; Tielens, A. G. G. M.

2012-12-01

Context. The interaction between stellar winds and the interstellar medium (ISM) can create complex bow shocks. The photometers on board the Herschel Space Observatory are ideally suited to studying the morphologies of these bow shocks. Aims: We aim to study the circumstellar environment and wind-ISM interaction of the nearest red supergiant, Betelgeuse. Methods.Herschel PACS images at 70, 100, and 160 μm and SPIRE images at 250, 350, and 500 μm were obtained by scanning the region around Betelgeuse. These data were complemented with ultraviolet GALEX data, near-infrared WISE data, and radio 21 cm GALFA-HI data. The observational properties of the bow shock structure were deduced from the data and compared with hydrodynamical simulations. Results: The infrared Herschel images of the environment around Betelgeuse are spectacular, showing the occurrence of multiple arcs at ~6-7' from the central target and the presence of a linear bar at ~9'. Remarkably, no large-scale instabilities are seen in the outer arcs and linear bar. The dust temperature in the outer arcs varies between 40 and 140 K, with the linear bar having the same colour temperature as the arcs. The inner envelope shows clear evidence of a non-homogeneous clumpy structure (beyond 15''), probably related to the giant convection cells of the outer atmosphere. The non-homogeneous distribution of the material even persists until the collision with the ISM. A strong variation in brightness of the inner clumps at a radius of ~2' suggests a drastic change in mean gas and dust density ~32 000 yr ago. Using hydrodynamical simulations, we try to explain the observed morphology of the bow shock around Betelgeuse. Conclusions: Different hypotheses, based on observational and theoretical constraints, are formulated to explain the origin of the multiple arcs and the linear bar and the fact that no large-scale instabilities are visible in the bow shock region. We infer that the two main ingredients for explaining these phenomena are a non-homogeneous mass-loss process and the influence of the Galactic magnetic field. The hydrodynamical simulations show that a warm interstellar medium, reflecting a warm neutral or partially ionized medium, or a higher temperature in the shocked wind also prevent the growth of strong instabilities. The linear bar is probably an interstellar structure illuminated by Betelgeuse itself. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices (including movies) are available in electronic form at http://www.aanda.org

Investigation of relationships between parameters of solar nano-flares and solar activity

NASA Astrophysics Data System (ADS)

Safari, Hossein; Javaherian, Mohsen; Kaki, Bardia

2016-07-01

Solar flares are one of the important coronal events which are originated in solar magnetic activity. They release lots of energy during the interstellar medium, right after the trigger. Flare prediction can play main role in avoiding eventual damages on the Earth. Here, to interpret solar large-scale events (e.g., flares), we investigate relationships between small-scale events (nano-flares) and large-scale events (e.g., flares). In our method, by using simulations of nano-flares based on Monte Carlo method, the intensity time series of nano-flares are simulated. Then, the solar full disk images taken at 171 angstrom recorded by SDO/AIA are employed. Some parts of the solar disk (quiet Sun (QS), coronal holes (CHs), and active regions (ARs)) are cropped and the time series of these regions are extracted. To compare the simulated intensity time series of nano-flares with the intensity time series of real data extracted from different parts of the Sun, the artificial neural networks is employed. Therefore, we are able to extract physical parameters of nano-flares like both kick and decay rate lifetime, and the power of their power-law distributions. The procedure of variations in the power value of power-law distributions within QS, CH is similar to AR. Thus, by observing the small part of the Sun, we can follow the procedure of solar activity.

Processing and Synthesis of Pre-Biotic Chemicals in Hypervelocity Impacts

NASA Technical Reports Server (NTRS)

Brickerhoff, W. B.; Managadze, G. G.; Chumikov, A. E.; Managadze, N. G.

2005-01-01

Hypervelocity impacts (HVIs) may have played a significant role in establishing the initial organic inventory for pre-biotic chemistry on the Earth and other planetary bodies. In addition to the delivery of organic compounds intact to planetary surfaces, generally at velocities below approx.20 km/s, HVIs also enable synthesis of new molecules. The cooling post-impact plasma plumes of HVIs in the interstellar medium (ISM), the protosolar nebula (PSN), and the early solar system comprise pervasive conditions for organic synthesis. Such plasma synthesis (PS) can operate over many length scales (from nm-scale dust to planets) and energy scales (from molecular rearrangement to atomization and recondensation). HVI experiments with the flexibility to probe the highest velocities and distinguish synthetic routes are a high priority to understand the relevance of PS to exobiology. We describe here recent studies of PS at small spatial scales and extremely high velocities with pulsed laser ablation (PLA). PLA can simulate the extreme plasma conditions generated in impacts of dust particles at speeds of up to 100 km/s or more. When applied to carbonaceous solids, new and pre-biotically relevant molecular species are formed with high efficiency [1,2].

Radiative heating of interstellar grains falling toward the solar nebula: 1-D diffusion calculations

NASA Technical Reports Server (NTRS)

Simonelli, D. P.; Pollack, J. B.; McKay, C. P.

1997-01-01

As the dense molecular cloud that was the precursor of our Solar System was collapsing to form a protosun and the surrounding solar-nebula accretion disk, infalling interstellar grains were heated much more effectively by radiation from the forming protosun than by radiation from the disk's accretion shock. Accordingly, we have estimated the temperatures experienced by these infalling grains using radiative diffusion calculations whose sole energy source is radiation from the protosun. Although the calculations are 1-dimensional, they make use of 2-D, cylindrically symmetric models of the density structure of a collapsing, rotating cloud. The temperature calculations also utilize recent models for the composition and radiative properties of interstellar grains (Pollack et al. 1994. Astrophys. J. 421, 615-639), thereby allowing us to estimate which grain species might have survived, intact, to the disk accretion shock and what accretion rates and molecular-cloud rotation rates aid that survival. Not surprisingly, we find that the large uncertainties in the free parameter values allow a wide range of grain-survival results: (1) For physically plausible high accretion rates or low rotation rates (which produce small accretion disks), all of the infalling grain species, even the refractory silicates and iron, will vaporize in the protosun's radiation field before reaching the disk accretion shock. (2) For equally plausible low accretion rates or high rotation rates (which produce large accretion disks), all non-ice species, even volatile organics, will survive intact to the disk accretion shock. These grain-survival conclusions are subject to several limitations which need to be addressed by future, more sophisticated radiative-transfer models. Nevertheless, our results can serve as useful inputs to models of the processing that interstellar grains undergo at the solar nebula's accretion shock, and thus help address the broader question of interstellar inheritance in the solar nebula and present Solar System. These results may also help constrain the size of the accretion disk: for example, if we require that the calculations produce partial survival of organic grains into the solar nebula, we infer that some material entered the disk intact at distances comparable to or greater than a few AU. Intriguingly, this is comparable to the heliocentric distance that separates the C-rich outer parts of the current Solar System from the C-poor inner regions.

Radiative heating of interstellar grains falling toward the solar nebula: 1-D diffusion calculations.

PubMed

Simonelli, D P; Pollack, J B; McKay, C P

1997-02-01

As the dense molecular cloud that was the precursor of our Solar System was collapsing to form a protosun and the surrounding solar-nebula accretion disk, infalling interstellar grains were heated much more effectively by radiation from the forming protosun than by radiation from the disk's accretion shock. Accordingly, we have estimated the temperatures experienced by these infalling grains using radiative diffusion calculations whose sole energy source is radiation from the protosun. Although the calculations are 1-dimensional, they make use of 2-D, cylindrically symmetric models of the density structure of a collapsing, rotating cloud. The temperature calculations also utilize recent models for the composition and radiative properties of interstellar grains (Pollack et al. 1994. Astrophys. J. 421, 615-639), thereby allowing us to estimate which grain species might have survived, intact, to the disk accretion shock and what accretion rates and molecular-cloud rotation rates aid that survival. Not surprisingly, we find that the large uncertainties in the free parameter values allow a wide range of grain-survival results: (1) For physically plausible high accretion rates or low rotation rates (which produce small accretion disks), all of the infalling grain species, even the refractory silicates and iron, will vaporize in the protosun's radiation field before reaching the disk accretion shock. (2) For equally plausible low accretion rates or high rotation rates (which produce large accretion disks), all non-ice species, even volatile organics, will survive intact to the disk accretion shock. These grain-survival conclusions are subject to several limitations which need to be addressed by future, more sophisticated radiative-transfer models. Nevertheless, our results can serve as useful inputs to models of the processing that interstellar grains undergo at the solar nebula's accretion shock, and thus help address the broader question of interstellar inheritance in the solar nebula and present Solar System. These results may also help constrain the size of the accretion disk: for example, if we require that the calculations produce partial survival of organic grains into the solar nebula, we infer that some material entered the disk intact at distances comparable to or greater than a few AU. Intriguingly, this is comparable to the heliocentric distance that separates the C-rich outer parts of the current Solar System from the C-poor inner regions.

The extinction and dust-to-gas structure of the planetary nebula NGC 7009 observed with MUSE

NASA Astrophysics Data System (ADS)

Walsh, J. R.; Monreal-Ibero, A.; Barlow, M. J.; Ueta, T.; Wesson, R.; Zijlstra, A. A.

2016-04-01

Context. Dust plays a significant role in planetary nebulae. Dust ejected with the gas in the asymptotic giant branch (AGB) phase is subject to the harsh environment of the planetary nebula (PN) while the star is evolving towards a white dwarf. Dust surviving the PN phase contributes to the dust content of the interstellar medium. Aims: The morphology of the internal dust extinction has been mapped for the first time in a PN, the bright nearby Galactic nebula NGC 7009. The morphologies of the gas, dust extinction and dust-to-gas ratio are compared to the structural features of the nebula. Methods: Emission line maps in H Balmer and Paschen lines were formed from analysis of MUSE cubes of NGC 7009 observed during science verification of the instrument. The measured electron temperature and density from the same cube were employed to predict the theoretical H line ratios and derive the extinction distribution across the nebula. After correction for the interstellar extinction to NGC 7009, the internal AV/NH has been mapped for the first time in a PN. Results: The extinction map of NGC 7009 has considerable structure, broadly corresponding to the morphological features of the nebula. The dust-to-gas ratio, AV/NH, increases from 0.7 times the interstellar value to >5 times from the centre towards the periphery of the ionized nebula. The integrated AV/NH is about 2× the mean ISM value. A large-scale feature in the extinction map is a wave, consisting of a crest and trough, at the rim of the inner shell. The nature of this feature is investigated and instrumental and physical causes considered; no convincing mechanisms were identified to produce this feature, other than AGB mass loss variations. Conclusions: Extinction mapping from H emission line imaging of PNe with MUSE provides a powerful tool for revealing the properties of internal dust and the dust-to-gas ratio. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 060.A-9347(A).

Disintegration of Dust Aggregates in Interstellar Shocks and the Lifetime of Dust Grains in the ISM

NASA Technical Reports Server (NTRS)

Dominik, C.; Jones, A. P.; Tielens, A. G. G. M.; Cuzzi, Jeff (Technical Monitor)

1994-01-01

Interstellar grains are destroyed by shock waves moving through the ISM. In fact, the destruction of grains may be so effective that it is difficult to explain the observed abundance of dust in the ISM as a steady state between input of grains from stellar sources and destruction of grains in shocks. This is especially a problem for the larger grains. Therefore, the dust grains must be protected in some way. Jones et al. have already considered coatings and the increased post-shock drag effects for low density grains. In molecular clouds and dense clouds, coagulation of grains is an important process, and the largest interstellar grains may indeed be aggregates of smaller grains rather than homogeneous particles. This may provide a means to protect the larger grains, in that, in moderate velocity grain-grain collisions in a shock the aggregates may disintegrate rather than be vaporized. The released small particles are more resilient to shock destruction (except in fast shocks) and may reform larger grains later, recovering the observed size distribution. We have developed a model for the binding forces in grain aggregates and apply this model to the collisions between an aggregate and fast small grains. We discuss the results in the light of statistical collision probabilities and grain life times.

The influence of the ionized medium on synchrotron emission in interstellar space.

NASA Technical Reports Server (NTRS)

Ramaty, R.

1972-01-01

The effect of the ionized gas on synchrotron emission in the interstellar medium is investigated. A detailed calculation of the synchrotron emissivity of cosmic electrons, assumed to have an isotropic pitch-angle distribution in a uniform magnetic field, is made as a function of frequency and observation angle with respect to the field. The results are presented both as a local emissivity and as an intensity, the latter obtained by neglecting free-free absorption in the interstellar medium and by assuming that the emissivity is constant along the line of sight. The comparison of these results with previous studies on the nature of the low-frequency turnover of the galactic nonthermal radio background reveals that, except if the component perpendicular to the line of sight of the interstellar magnetic field is small (less than 1 microgauss), or if the cosmic-ray electron spectrum is cut off at energies below a few hundred MeV, the suppression of synchrotron emission by the ambient electrons has in general a lesser effect than free-free absorption by these electrons, and that in some cases this suppression effect is almost entirely negligible.

Solid State Pathways towards Molecular Complexity in Space

NASA Astrophysics Data System (ADS)

Linnartz, Harold; Bossa, Jean-Baptiste; Bouwman, Jordy; Cuppen, Herma M.; Cuylle, Steven H.; van Dishoeck, Ewine F.; Fayolle, Edith C.; Fedoseev, Gleb; Fuchs, Guido W.; Ioppolo, Sergio; Isokoski, Karoliina; Lamberts, Thanja; Öberg, Karin I.; Romanzin, Claire; Tenenbaum, Emily; Zhen, Junfeng

2011-12-01

It has been a long standing problem in astrochemistry to explain how molecules can form in a highly dilute environment such as the interstellar medium. In the last decennium more and more evidence has been found that the observed mix of small and complex, stable and highly transient species in space is the cumulative result of gas phase and solid state reactions as well as gas-grain interactions. Solid state reactions on icy dust grains are specifically found to play an important role in the formation of the more complex ``organic'' compounds. In order to investigate the underlying physical and chemical processes detailed laboratory based experiments are needed that simulate surface reactions triggered by processes as different as thermal heating, photon (UV) irradiation and particle (atom, cosmic ray, electron) bombardment of interstellar ice analogues. Here, some of the latest research performed in the Sackler Laboratory for Astrophysics in Leiden, the Netherlands is reviewed. The focus is on hydrogenation, i.e., H-atom addition reactions and vacuum ultraviolet irradiation of interstellar ice analogues at astronomically relevant temperatures. It is shown that solid state processes are crucial in the chemical evolution of the interstellar medium, providing pathways towards molecular complexity in space.

κ -distributed protons in the solar wind and their charge-exchange coupling to energetic hydrogen

DOE PAGES

Heerikhuisen, J.; Zirnstein, Eric; Pogorelov, Nikolai

2015-03-16

The interaction between the solar wind and the interstellar medium represents a collision between two plasma flows, resulting in a heliosphere with an extended tail. While the solar wind is mostly ionized material from the corona, the interstellar medium is only partially ionized. The ion and neutral populations are coupled through charge-exchange collisions that operate on length scales of tens to hundreds of astronomical units. About half the interstellar hydrogen flows into the heliosphere where it may charge-exchange with solar wind protons. This process gives rise to a nonthermal proton, known as a pickup ion, which joins the plasma. Inmore » this paper we investigate the effects of approximating the total ion distribution of the subsonic solar wind as a generalized Lorentzian, or κ distribution, using an MHD neutral code. We illustrate the effect different values of the κ parameter have on both the structure of the heliosphere and the energetic neutral atom flux at 1 AU. We find that using a κ distribution in our simulations yields levels of energetic neutral atom flux that are within a factor of about 2 or 3 over the IBEX-Hi range of energies from 0.5 to 6 keV. In conclusion, while the presence of a suprathermal tail in the proton distribution leads to the production of high-energy neutrals, the sharp decline in the charge-exchange cross section around 10 keV mitigates the enhanced transfer of energy from the ions to the neutrals that might otherwise be expected.« less

A note on compressibility and energy cascade in turbulent molecular clouds

NASA Technical Reports Server (NTRS)

Fleck, R. C., Jr.

1983-01-01

Observed velocity-size correlations are reexamined in the light of an improved theory of turbulent energy cascade that is developed. It is shown that observed velocity-size correlations cannot be compared with the Kolmogorov law, which is based on incompressible turbulent flow. The fact that the log v-log(l/rho) scaling law (v the turbulent velocity, l the associated region size, and rho the fluid density) predicted for compressible energy cascade is always steeper than that observed in molecular clouds indicates the injection rather than the dissipation of mechanical energy at smaller scales of motion. It is also shown that the concept of strict energy cascade may not be generally applicable in the interstellar medium. The agreement between theory and observation turns out to be best for small cool clouds and cloud cores, suggesting that, for these regions at least, the dominant process in establishing the observed v-l-rho correlation is a turbulent energy cascade.

Observing Interstellar and Intergalactic Magnetic Fields

NASA Astrophysics Data System (ADS)

Han, J. L.

2017-08-01

Observational results of interstellar and intergalactic magnetic fields are reviewed, including the fields in supernova remnants and loops, interstellar filaments and clouds, Hii regions and bubbles, the Milky Way and nearby galaxies, galaxy clusters, and the cosmic web. A variety of approaches are used to investigate these fields. The orientations of magnetic fields in interstellar filaments and molecular clouds are traced by polarized thermal dust emission and starlight polarization. The field strengths and directions along the line of sight in dense clouds and cores are measured by Zeeman splitting of emission or absorption lines. The large-scale magnetic fields in the Milky Way have been best probed by Faraday rotation measures of a large number of pulsars and extragalactic radio sources. The coherent Galactic magnetic fields are found to follow the spiral arms and have their direction reversals in arms and interarm regions in the disk. The azimuthal fields in the halo reverse their directions below and above the Galactic plane. The orientations of organized magnetic fields in nearby galaxies have been observed through polarized synchrotron emission. Magnetic fields in the intracluster medium have been indicated by diffuse radio halos, polarized radio relics, and Faraday rotations of embedded radio galaxies and background sources. Sparse evidence for very weak magnetic fields in the cosmic web is the detection of the faint radio bridge between the Coma cluster and A1367. Future observations should aim at the 3D tomography of the large-scale coherent magnetic fields in our Galaxy and nearby galaxies, a better description of intracluster field properties, and firm detections of intergalactic magnetic fields in the cosmic web.

Contribution of Massive Stars to the Production of Neutron Capture Elements

NASA Astrophysics Data System (ADS)

Federman, Steven

2010-09-01

Elements beyond the Fe-peak must be synthesized through neutron-capture processes. With the aim of understanding the contribution of massive stars to the synthesis of neutron-capture elements during the current epoch, we propose an archival survey of interstellar arsenic, cadmium, tin, and lead. Nucleosynthesis via the weak slow process and the rapid process are the routes involving massive stars, while the main slow process arises from the evolution of low-mass stars. Ultraviolet lines for the dominant ions for each element will be used to extract interstellar abundances. The survey involves about forty sight lines, many of which are associated with regions of massive star formation shaped by core-collapse supernovae {SNe II}. The sample will increase the number of published determinations by factors of 2 to 5. HST spectra are the only means for determining the elemental abundances for this set of species in diffuse interstellar clouds. The survey contains directions that are both molecule poor and molecule rich, thereby enabling us to examine the overall level of depletion onto grains as a function of gas density. Complementary laboratory determinations of oscillator strengths will place the interstellar measurements on an absolute scale. The results from the proposed study will be combined with published interstellar abundances for other neutron capture elements and the suite of measurements will be compared to results from stars throughout the history of the Galaxy.

Update on IBEX and the outer boundary of the space radiation environment

NASA Astrophysics Data System (ADS)

McComas, D. J.; IBEX Science Team

2012-11-01

The Interstellar Boundary Explorer (IBEX) mission has been remotely observing the global interaction of our heliosphere with the local interstellar medium for over three years. Initially, IBEX generated the first all-sky maps of Energetic Neutral Atoms (ENAs) emanating in from the boundaries of our heliosphere over the energy range from ˜0.1-6 keV. Using these observations, the IBEX team discovered a smoothly varying, globally distributed ENA flux overlaid by a narrow "ribbon" of significantly enhanced ENA emissions. Since the initial publication of these results in a special issue of Science magazine (November 2009), IBEX has completed five more energy-resolved sets of sky maps and discovered small but important time variations in the interaction, separated the ribbon from globally distributed ENA fluxes, measured the energy spectral shape and inferred ion source temperatures, and carried out many other observational and theoretical studies of the outer heliosphere. In a second major area of observations - direct measurements of Interstellar Neutral (ISN) atoms - just published, IBEX observations of ISN He atoms show that the speed and direction (the motion of the heliosphere with respect to the interstellar medium) is slower and from a somewhat different direction than that thought from prior Ulysses observations. These observations also show evidence for a previously unknown and unanticipated secondary population of Helium. In addition, IBEX is providing the first direct quantitative measurements of the ISN H parameters and the first direct measurements of interstellar Ne and the interstellar Neon/Oxygen abundance ratio; this ratio is significantly different than the solar abundance ratio. Finally, IBEX was recently maneuvered into a unique, long-term stable orbit, which has a very low radiation environment and requires no orbit maintenance. Thus, IBEX will likely continue to provide revolutionary observations of the outer heliosphere and local interstellar medium for many years to come.

Some new reaction pathways for the formation of cytosine in interstellar space - A quantum chemical study

NASA Astrophysics Data System (ADS)

Gupta, V. P.; Tandon, Poonam; Mishra, Priti

2013-03-01

The detection of nucleic acid bases in carbonaceous meteorites suggests that their formation and survival is possible outside of the Earth. Small N-heterocycles, including pyrimidine, purines and nucleobases, have been extensively sought in the interstellar medium. It has been suggested theoretically that reactions between some interstellar molecules may lead to the formation of cytosine, uracil and thymine though these processes involve significantly high potential barriers. We attempted therefore to use quantum chemical techniques to explore if cytosine can possibly form in the interstellar space by radical-radical and radical-molecule interaction schemes, both in the gas phase and in the grains, through barrier-less or low barrier pathways. Results of DFT calculations for the formation of cytosine starting from some of the simple molecules and radicals detected in the interstellar space are being reported. Global and local descriptors such as molecular hardness, softness and electrophilicity, and condensed Fukui functions and local philicity indices were used to understand the mechanistic aspects of chemical reaction. The presence and nature of weak bonds in the molecules and transition states formed during the reaction process have been ascertained using Bader's quantum theory of atoms in molecules (QTAIMs). Two exothermic reaction pathways starting from propynylidyne (CCCH) and cyanoacetylene (HCCCN), respectively, have been identified. While the first reaction path is found to be totally exothermic, it involves a barrier of 12.5 kcal/mol in the gas phase against the lowest value of about 32 kcal/mol reported in the literature. The second path is both exothermic and barrier-less. The later has, therefore, a greater probability of occurrence in the cold interstellar clouds (10-50 K).

Application of Stochastic and Deterministic Approaches to Modeling Interstellar Chemistry

NASA Astrophysics Data System (ADS)

Pei, Yezhe

This work is about simulations of interstellar chemistry using the deterministic rate equation (RE) method and the stochastic moment equation (ME) method. Primordial metal-poor interstellar medium (ISM) is of our interest and the socalled “Population-II” stars could have been formed in this environment during the “Epoch of Reionization” in the baby universe. We build a gas phase model using the RE scheme to describe the ionization-powered interstellar chemistry. We demonstrate that OH replaces CO as the most abundant metal-bearing molecule in such interstellar clouds of the early universe. Grain surface reactions play an important role in the studies of astrochemistry. But the lack of an accurate yet effective simulation method still presents a challenge, especially for large, practical gas-grain system. We develop a hybrid scheme of moment equations and rate equations (HMR) for large gas-grain network to model astrochemical reactions in the interstellar clouds. Specifically, we have used a large chemical gas-grain model, with stochastic moment equations to treat the surface chemistry and deterministic rate equations to treat the gas phase chemistry, to simulate astrochemical systems as of the ISM in the Milky Way, the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). We compare the results to those of pure rate equations and modified rate equations and present a discussion about how moment equations improve our theoretical modeling and how the abundances of the assorted species are changed by varied metallicity. We also model the observed composition of H2O, CO and CO2 ices toward Young Stellar Objects in the LMC and show that the HMR method gives a better match to the observation than the pure RE method.

Scaling laws of passive-scalar diffusion in the interstellar medium

NASA Astrophysics Data System (ADS)

Colbrook, Matthew J.; Ma, Xiangcheng; Hopkins, Philip F.; Squire, Jonathan

2017-05-01

Passive-scalar mixing (metals, molecules, etc.) in the turbulent interstellar medium (ISM) is critical for abundance patterns of stars and clusters, galaxy and star formation, and cooling from the circumgalactic medium. However, the fundamental scaling laws remain poorly understood in the highly supersonic, magnetized, shearing regime relevant for the ISM. We therefore study the full scaling laws governing passive-scalar transport in idealized simulations of supersonic turbulence. Using simple phenomenological arguments for the variation of diffusivity with scale based on Richardson diffusion, we propose a simple fractional diffusion equation to describe the turbulent advection of an initial passive scalar distribution. These predictions agree well with the measurements from simulations, and vary with turbulent Mach number in the expected manner, remaining valid even in the presence of a large-scale shear flow (e.g. rotation in a galactic disc). The evolution of the scalar distribution is not the same as obtained using simple, constant 'effective diffusivity' as in Smagorinsky models, because the scale dependence of turbulent transport means an initially Gaussian distribution quickly develops highly non-Gaussian tails. We also emphasize that these are mean scalings that apply only to ensemble behaviours (assuming many different, random scalar injection sites): individual Lagrangian 'patches' remain coherent (poorly mixed) and simply advect for a large number of turbulent flow-crossing times.

Three-phase Interstellar Medium in Galaxies Resolving Evolution with Star Formation and Supernova Feedback (TIGRESS): Algorithms, Fiducial Model, and Convergence

NASA Astrophysics Data System (ADS)

Kim, Chang-Goo; Ostriker, Eve C.

2017-09-01

We introduce TIGRESS, a novel framework for multi-physics numerical simulations of the star-forming interstellar medium (ISM) implemented in the Athena MHD code. The algorithms of TIGRESS are designed to spatially and temporally resolve key physical features, including: (1) the gravitational collapse and ongoing accretion of gas that leads to star formation in clusters; (2) the explosions of supernovae (SNe), both near their progenitor birth sites and from runaway OB stars, with time delays relative to star formation determined by population synthesis; (3) explicit evolution of SN remnants prior to the onset of cooling, which leads to the creation of the hot ISM; (4) photoelectric heating of the warm and cold phases of the ISM that tracks the time-dependent ambient FUV field from the young cluster population; (5) large-scale galactic differential rotation, which leads to epicyclic motion and shears out overdense structures, limiting large-scale gravitational collapse; (6) accurate evolution of magnetic fields, which can be important for vertical support of the ISM disk as well as angular momentum transport. We present tests of the newly implemented physics modules, and demonstrate application of TIGRESS in a fiducial model representing the solar neighborhood environment. We use a resolution study to demonstrate convergence and evaluate the minimum resolution {{Δ }}x required to correctly recover several ISM properties, including the star formation rate, wind mass-loss rate, disk scale height, turbulent and Alfvénic velocity dispersions, and volume fractions of warm and hot phases. For the solar neighborhood model, all these ISM properties are converged at {{Δ }}x≤slant 8 {pc}.

Wrinkle structures—a critical review

NASA Astrophysics Data System (ADS)

Porada, Hubertus; Bouougri, El Hafid

2007-04-01

In this paper, a variety of so-called 'wrinkle structures' is reviewed in an attempt to help distinguish between crinkly decorations arising from physical processes that acted on siliciclastic bedding surfaces, and true microbially induced 'wrinkle structures'. Two types of small-scale, microbially induced sedimentary structures are prominent due to their distinct geometry and mode of occurrence: (1) 'elephant skin' textures, characterized by reticulate patterns of sharp-crested ridges forming mm- to cm-scale polygons, occurring on argillite or argillaceous veneers above fine-grained sandstone and likely reflecting growth structures of microbial, mats (2) 'Kinneyia' structures, characterized by mm-scale flat-topped, winding ridges and intervening troughs and pits, sometimes resembling small-scale interference ripples. 'Kinneyia' structures usually occur on upper surfaces of siltstone/sandstone beds, themselves frequently event deposits, and are thought to have formed beneath microbial mats. Additionally, more linear variations of mat growth structures, partly resembling small-scale 'α-petees' may be developed. Finally, some wrinkly structures resulting from tractional mat deformation or mat slumping are occasionally preserved. These may appear as arcuate belts of non-penetrative, small-scale folds or as wrinkled bulges on otherwise flat surfaces. 'Wrinkle structures' as indicators for the former presence of mats gain in importance if other mat-related structures are additionally observed in the same clastic succession, e.g. 'sand chips' (sandy intraclasts) or spindle-shaped or sinuously curved to circular sand cracks, frequently combined in networks. Furthermore, appropriate lithologies and facies are required. For instance, if compared with the distribution of modern cohesive microbial mats, laminated siltstone/argillite with intercalated siltstone/sandstone beds representing event deposits in tidal flat successions would be compatible with microbial mat development. Within a variety of physically induced small-scale wrinkly structures, miniature load structures may, above all, be misinterpreted as microbially induced 'wrinkle structures', due to their similar size and appearance, and their comparatively frequent occurrence.

Cosmic Ray Anisotropies and Magnetic Turbulence Beyond the Heliopause

NASA Astrophysics Data System (ADS)

Florinski, V. A.

2016-12-01

The very local interstellar medium (VLISM), including the outer heliosheath, represents a quiet, almost laminar environment for cosmic-ray propagation. The dominant scale of magnetic-field fluctuations in the VLISM is about a million astronomical units - three orders of magnitude larger than the size of the heliosphere. Under these conditions the transport of cosmic rays is governed mainly by three effects: (a) draping of the magnetic field around the heliopause, (b) bending of magnetic field lines representing VLISM turbulence at large scales, and (c) local deformation of magnetic field lines by disturbances injected into VLISM as a result of solar-wind merged interaction regions impinging on the heliopause. Using analytic and computer-based models of the outer heliosheath magnetic field and phase space trajectory integration techniques to simulate charged particle transport, the relationship between the magnetic field properties and hundred MeV galactic cosmic ray ion anisotropies is investigated. It is demonstrated that anisotropy measurements can be used to deduce the amplitude and spatial scale of interstellar magnetic turbulence.

Experimental and Theoretical Studies of Interstellar Grains. Ph.D. Thesis - Maryland Univ., College Park, 1982

NASA Technical Reports Server (NTRS)

Nuth, J. A., III

1981-01-01

Steady state vibrational populations of SiO and CO in dilute black body radiation fields were calculated as a function of total pressure, kinetic temperature and chemical composition of the gas. Approximate calculations for polyatomic molecules are presented. Vibrational disequilibrium becomes increasingly significant as total pressure and radiation density decrease. Many regions of postulated grain formation are found to be far from thermal equilibrium before the onset of nucleation. Calculations based upon classical nucleation theory or equilibrium thermodynamics are expected to be of dubious value in such regions. Laboratory measurements of the extinction of small iron and magnetite grains were made from 195 nm to 830 nm and found to be consistent with predictions based upon published optical constants. This implies that small iron particles are not responsible for the 220 nm interstellar extinction features. Additional measurements are discussed.

Helical structures in a Rosette elephant trunk

NASA Astrophysics Data System (ADS)

Carlqvist, Per; Kristen, Helmuth; Gahm, Gosta F.

1998-04-01

We discuss small-scale, helical, interstellar filaments on the basis of optical observations of an elephant trunk in the Rosette nebula. The trunk studied is composed of a number of sinusoidal or serpentine-like dark filaments, preferentially in the outer part of the trunk, where their wavelength is 7-9 times the trunk radius. The diameters are down to the limit of resolution of 1.0 arcsec, corresponding to 1600 au, and ranging up to about 6400 au. At some positions filament crossings give rise to enhanced extinction. We suggest that the sinusoidal filaments are helices lined up by magnetic fields. We derive average extinctions of 0.5-1.0 mag in the filaments, implying molecular densities of n_H2 ~ 10(4) cm(-3) . From existing data on the Rosette HiI region, we conclude that the surrounding kinetic and dynamic pressure and the background radiation field suffice to balance even the denser filaments and to exert drag forces on the trunk as a whole, consistent with evidence of stretching of the trunk. The helical magnetic structures imply the presence of electric currents along the trunk axis. These currents should form a nearly force-free geometry and are consistent with a model consisting of 4-7 helical cables on the surface of a cylinder and which produce the observed wavelength of the helices. We suggest that the Rosette elephant trunks form an interconnected system of rope-like structures which are relics from filamentary skeletons of magnetic fields in the primordial cloud. Based on observations collected at the Nordic Optical Telescope, La Palma, Spain

Pre-Biological Evolution of Organic Matter in the Universe

NASA Astrophysics Data System (ADS)

Wiebe, D. Z.

2017-05-01

Discovery of interstellar molecules has become one of the most prominent findings of 20th century. Initially (since late 1930-ies) only simple two-atom compounds have been known. However, the rapid development of radioastronomy during post-war years has allowed expanding this list significantly. Now, the number of known interstellar and circumstellar molecules approaches two hundred (not counting isomers and isotopologues). Among them we see both simple and quite complex molecules. The largest molecules with solid identification consist of 12 atoms (CH3OC2H5, C3H7CN). Nearly all molecules with more than five atoms represent are organic. More than once even discovery of the simplest amino acid (glycine) in the interstellar medium had been reported. While later all these reports has been refuted, there is no doubt that this is a purely technical problem, and there are no fundamental obstacles on a pathway to interstellar synthesis of simplest amino acids. Definitely, even more complex organic structures are present in the interstellar medium, like fullerenes and some kind of aromatic particles. Recently, this diversity quite often became an incentive to suggest that organic species might have arrived to Earth (and other forming planets) in a "ready-to-use" form. However, one has to remember that numerous factors causing effective molecule destruction are in action in the interstellar medium, in the vicinity of young stars, and in protoplanetary disks.

Dust evolution, a global view: III. Core/mantle grains, organic nano-globules, comets and surface chemistry

PubMed Central

2016-01-01

Within the framework of The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS), this work explores the surface processes and chemistry relating to core/mantle interstellar and cometary grain structures and their influence on the nature of these fascinating particles. It appears that a realistic consideration of the nature and chemical reactivity of interstellar grain surfaces could self-consistently and within a coherent framework explain: the anomalous oxygen depletion, the nature of the CO dark gas, the formation of ‘polar ice’ mantles, the red wing on the 3 μm water ice band, the basis for the O-rich chemistry observed in hot cores, the origin of organic nano-globules and the 3.2 μm ‘carbonyl’ absorption band observed in comet reflectance spectra. It is proposed that the reaction of gas phase species with carbonaceous a-C(:H) grain surfaces in the interstellar medium, in particular the incorporation of atomic oxygen into grain surfaces in epoxide functional groups, is the key to explaining these observations. Thus, the chemistry of cosmic dust is much more intimately related with that of the interstellar gas than has previously been considered. The current models for interstellar gas and dust chemistry will therefore most likely need to be fundamentally modified to include these new grain surface processes. PMID:28083090

Hydrocarbons in the ISM: Their Evolution and the Grain-to-Molecule Transition

NASA Astrophysics Data System (ADS)

Jones, Anthony P.

The evolution of hydrocarbon grains in the ISM is determined, principally, by the effects of photo-processing (annealing) which lead to a progressive loss of hydrogen from the structure and an associated 'graphitisation' of the material. Eventually this 'graphitisation' results in a low-density, highly aromatic material that can disaggregate into its aromatic-rich molecular components. These changes are followed through the use of an extended random covalent network (RCN) model for the hydrocarbon structure. This type of 'top down' process could be a significant source of the large molecular infrared band carriers in photon dominated regions. On the basis of this simple model there should thus be a relationship between the small grain and large molecule infrared emission bands across, and within, astrophysical boundaries such as photo-dissociation regions. 1. Introduction Carbon is the most abundant dust-forming element in the ISM and a large fraction of this carbon is in the form of grains comprised, principally, of hydrocarbon materials, including those where the hydrogen content is minimal. Interstellar hydrocarbon grains include: graphite, hydrogenated amorphous aliphatic and/or aromatic hydrocarbons (a-C, a-C:H) and (nano)diamond. These hydrocarbon dusts play a pivotal role in determining, amongst other things, the interstellar extinction, the dust thermal emission and the photo-electric heating of the gas in the ISM. 2. Hydrocarbon grains in the ISM Hydrocarbon grains are formed in the circumstellar shells around C-rich evolved stars, in supernova ejecta and also in the ISM itself via accretion and solid-state chemistry. The physical and chemical properties of hydrocarbon grains are indeed complex and vary in response to the ambient conditions (density, temperature, radiation field, ...). For example they can undergo both chemical and physical processing (growth and changes in chemical composition through accretion and reaction, erosion via inertial or chemi-sputtering, photo-darkening or 'graphitisation', photo-disruption in intense radiation fields and fragmentation in interstellar shock waves). Recently, using laboratory simulations of carbon dust analogues, Dartois, Muñoz Caro, Deboffle, et al. (2004,2005) have shown that hydrogen-rich (> 50 atomic % H) hydrocarbon solids can explain the observed interstellar absorption bands at 3.4, 6.85 and 7.25 μm. They also show that the thermal annealing of this material is accompanied by an increase in the aromatic component, i.e., a 'graphitisation'. Such a transformation and evolution of interstellar hydrocarbons was proposed by Duley, Jones & Williams (1989) and Jones, Duley & Wiliams (1990). This photon-driven process, acting on the hydrogen-rich hydrocarbon grains in the ISM that originate predominantly from carbon-rich evolved stars, will lead to a progressive loss of hydrogen and an associated 'graphitisation' and 'openingup' of the structure. The end point of the 'graphitisation' process is then a low-density material that will disaggregate into its constituent aromatic molecular components and the necessarily-associated sp3 and sp2 carbon and hydrogen atom bridging structures, e.g., Duley (2000), Petrie, Stranger & Duley (2003). In this work we follow the 'graphitisation' process using a random covalent network (RCN) approach that extends the work of Jones (1990). To summarise, the RCN model characterises a series of hydrocarbons based upon the sp3/sp2 carbon atom ratio (R) and the atomic fraction of hydrogen (XH) within the material. This model allows an essentially one-parameter determination of the a-C:H structure and a prediction of the major infrared bands. We find that the large hydrocarbon grains, with temperatures in equilibrium with the local radiation field, will be rather hydrogen-rich and that the smaller grains, which undergo stochastic heating to high temperatures, will be converted into hydrogen-poorer and more graphitic materials. The photo-fragmentation of the smaller aromatic grains can be an important source of molecular aromatic species. The infrared spectrum of a given RCN hydrocarbon depends, principally, upon only its hydrogen content XH. The spectra predicted from this RCN model can then be compared with the interstellar absorption and emission bands in the 32 μm range. The compositional changes of a-C:H, and its constituent 'molecular' components, can then be mapped across a given region. 3. Conclusions The physics and chemistry of hydrocarbon grains is complex. Interstellar hydrocarbon grains will be a mixture of many different forms arising from many different sources and modified in many different regions of the ISM. Nevertheless, we can appreciate how this complex material evolves chemically, structurally and physically as a function of the ambient conditions through the use of a rather simple, and extended, RCN model. The transition/evolution of hydrocarbons in the ISM is, generally, from hydrogen-rich a-C:H, in the form of large grains formed around evolved stars, through to smaller, hydrogen-poor, low-density, aromatic a-C:H materials. The subsequent photo-fragmentation of the small aromatic grains could then be the origin of the aromatic emission band carriers within the ISM. We find that the spectral and physical properties of hydrocarbons in the ISM vary in a systematic way across, and also within, astrophysical environments. However, the history of the particles, prior to their incorporation into a given region, could also play a major role in determining their physical properties in that region. Thus, the evolution of hydrocarbon grains in the ISM will be size-, time- and history-dependent. It is therefore necessary that dust models take into account this complexity in predicting the properties of hydrocarbons in the ISM.

Gas-Phase Ion Chemistry in Interstellar, Circumstellar, and Planetary Environments

NASA Astrophysics Data System (ADS)

Demarais, Nicholas J.

In the last century, astronomers, physicists, and chemists have shown that the environments of space are complex. Although we have learned a great amount about the interstellar medium, circumstellar medium, and atmospheres of other planets and moons, many mysteries still remain unsolved. The cooperation of astronomers, modelers, and chemists has lead to the detection of over 180 molecules in the interstellar and circumstellar medium, and the evolution of the new scientific field of astrochemistry. Gas-phase ion chemistry can determine the stability of ions in these complex environments, provide chemical networks, and guide searches for new interstellar molecules. Using the flowing afterglow-selected ion flow tube (FA-SIFT), we have characterized the reactions of positive and negative ions that are important in a variety of astrochemical environments. The detection of CF+ in photodissociation regions highlights the importance of fluorinated species in the interstellar medium. The viability of CF+ as a possible diffuse interstellar band (DIB) carrier is discussed as related to reactions with neutral molecules in various interstellar conditions; the reactions of CF+ with twenty-two molecules of interstellar relevance were investigated. The chemical reactions of HCNH+ with H2, CH 4, C2H2, and C2H4 were reexamined to provide insight into the overprediction of HCNH+ in Titan's ionosphere by current astrochemical models. In addition, this work suggests other chemical reactions that should be included in the current models to fully describe the destruction rates of HCNH+ in Titan's ionosphere. The reactions of polycyclic aromatic hydrocarbon (PAH) ions with H atoms and other small molecules were carried out to determine the stability of these species. In diffuse regions, where the photon flux is high, PAH cations are the dominant ionization state. This work continues our previous research to include PAHs of differing geometries as well as nitrogen-containing PAHs. Extension to larger PAH cations was made possible by the integration of the laser induced acoustic desorption (LIAD) source with the FA-SIFT. In addition, in dense environments, where the photon flux is low, anionic PAHs may exist. The detection of negative ions in the past 10 years has highlighted the importance of their inclusion in astrochemical models. We have investigated the chemistry of deprotonated PAHs with molecules of interstellar relevance to determine their chemical stability in dense regions of the interstellar and circumstellar medium. In addition to PAH anions, H- is an important species in dense interstellar environments. While the reaction of hydride anion has been recognized as a critical mechanism in the initial cooling immediately after the Big Bang, H- + H → H2 + e-, chemistry with neutral molecules was largely unknown. The chemistry of H- with various classes of organic molecules was investigated and conclusions are drawn based on reaction mechanisms.

Organic compounds in circumstellar and interstellar environments.

PubMed

Kwok, Sun

2015-06-01

Recent research has discovered that complex organic matter is prevalent throughout the Universe. In the Solar System, it is found in meteorites, comets, interplanetary dust particles, and planetary satellites. Spectroscopic signatures of organics with aromatic/aliphatic structures are also found in stellar ejecta, diffuse interstellar medium, and external galaxies. From space infrared spectroscopic observations, we have found that complex organics can be synthesized in the late stages of stellar evolution. Shortly after the nuclear synthesis of the element carbon, organic gas-phase molecules are formed in the stellar winds, which later condense into solid organic particles. This organic synthesis occurs over very short time scales of about a thousand years. In order to determine the chemical structures of these stellar organics, comparisons are made with particles produced in the laboratory. Using the technique of chemical vapor deposition, artificial organic particles have been created by injecting energy into gas-phase hydrocarbon molecules. These comparisons led us to believe that the stellar organics are best described as amorphous carbonaceous nanoparticles with mixed aromatic and aliphatic components. The chemical structures of the stellar organics show strong similarity to the insoluble organic matter found in meteorites. Isotopic analysis of meteorites and interplanetary dust collected in the upper atmospheres have revealed the presence of pre-solar grains similar to those formed in old stars. This provides a direct link between star dust and the Solar System and raises the possibility that the early Solar System was chemically enriched by stellar ejecta with the potential of influencing the origin of life on Earth.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Giacinti, Gwenael; Kirk, John G.

We calculate the large-scale cosmic-ray (CR) anisotropies predicted for a range of Goldreich–Sridhar (GS) and isotropic models of interstellar turbulence, and compare them with IceTop data. In general, the predicted CR anisotropy is not a pure dipole; the cold spots reported at 400 TeV and 2 PeV are consistent with a GS model that contains a smooth deficit of parallel-propagating waves and a broad resonance function, though some other possibilities cannot, as yet, be ruled out. In particular, isotropic fast magnetosonic wave turbulence can match the observations at high energy, but cannot accommodate an energy dependence in the shape ofmore » the CR anisotropy. Our findings suggest that improved data on the large-scale CR anisotropy could provide a valuable probe of the properties—notably the power-spectrum—of the interstellar turbulence within a few tens of parsecs from Earth.« less

Formation of cyanoallene (buta-2, 3-dienenitrile) in the interstellar medium: a quantum chemical and spectroscopic study

NASA Astrophysics Data System (ADS)

Singh, Amresh; Shivani; Misra, Alka; Tandon, Poonam

2014-03-01

The interstellar medium, filling the vast space between stars, is a rich reservoir of molecular material ranging from simple diatomic molecules to more complex, astrobiologically important molecules such as vinylcyanide, methylcyanodiaccetylene, cyanoallene, etc. Interstellar molecular cyanoallene is one of the most stable isomers of methylcynoacetylene. An attempt has been made to explore the possibility of forming cyanoallene in interstellar space by radical-radical and radical-molecule interaction schemes in the gaseous phase. The formation of cyanoallene starting from some simple, neutral interstellar molecules and radicals has been studied using density functional theory. The reaction energies and structures of the reactants and products show that the formation of cyanoallene is possible in the gaseous phase. Both of the considered reaction paths are totally exothermic and barrierless, thus giving rise to a high probability of occurrence. Rate constants for each step in the formation process of cyanoallene in both the reaction paths are estimated. A full vibrational analysis has been attempted for cyanoallene in the harmonic and anharmonic approximations. Anharmonic spectroscopic parameters such as rotational constants, rotation-vibration coupling constants and centrifugal distortion constants have been calculated.

The Interstellar Heliopause Probe: Heliospheric Boundary Explorer Mission to the Interstellar Medium

NASA Astrophysics Data System (ADS)

Wimmer-Schweingruber, Robert F.; McNutt, Ralph

2009-04-01

The Sun, driving a supersonic solar wind, cuts out of the local interstellar medium a giant plasma bubble, the heliosphere. ESA, jointly with NASA, has had an important role in the development of our current understanding of the Suns’ immediate neighborhood. Ulysses is the only spacecraft exploring the third, out-of-ecliptic dimension, while SOHO has allowed us to better understand the influence of the Sun and to image the glow of interstellar matter in the heliosphere. Voyager 1 has recently encountered the innermost boundary of this plasma bubble, the termination shock, and is returning exciting yet puzzling data of this remote region. The next logical step is to leave the heliosphere and to thereby map out in unprecedented detail the structure of the outer heliosphere and its boundaries, the termination shock, the heliosheath, the heliopause, and, after leaving the heliosphere, to discover the true nature of the hydrogen wall, the bow shock, and the local interstellar medium beyond. This will greatly advance our understanding of the heliosphere that is the best-known example for astrospheres as found around other stars. Thus, IHP/HEX will allow us to discover, explore, and understand fundamental astrophysical processes in the largest accessible plasma laboratory, the heliosphere.

EVALUATING THE MORPHOLOGY OF THE LOCAL INTERSTELLAR MEDIUM: USING NEW DATA TO DISTINGUISH BETWEEN MULTIPLE DISCRETE CLOUDS AND A CONTINUOUS MEDIUM

DOE Office of Scientific and Technical Information (OSTI.GOV)

Redfield, Seth; Linsky, Jeffrey L., E-mail: sredfield@wesleyan.edu, E-mail: jlinsky@jila.colorado.edu

Ultraviolet and optical spectra of interstellar gas along the lines of sight to nearby stars have been interpreted by Redfield and Linsky and previous studies as a set of discrete warm, partially ionized clouds each with a different flow vector, temperature, and metal depletion. Recently, Gry and Jenkins proposed a fundamentally different model consisting of a single cloud with nonrigid flows filling space out to 9 pc from the Sun that they propose better describes the local ISM. Here we test these fundamentally different morphological models against the spatially unbiased Malamut et al. spectroscopic data set, and find that themore » multiple cloud morphology model provides a better fit to both the new and old data sets. The detection of three or more velocity components along the lines of sight to many nearby stars, the presence of nearby scattering screens, the observed thin elongated structures of warm interstellar gas, and the likely presence of strong interstellar magnetic fields also support the multiple cloud model. The detection and identification of intercloud gas and the measurement of neutral hydrogen density in clouds beyond the Local Interstellar Cloud could provide future morphological tests.« less

Pulsar Observations of Extreme Scattering Events

NASA Astrophysics Data System (ADS)

Coles, W. A.; Kerr, M.; Shannon, R. M.; Hobbs, G. B.; Manchester, R. N.; You, X.-P.; Bailes, M.; Bhat, N. D. R.; Burke-Spolaor, S.; Dai, S.; Keith, M. J.; Levin, Y.; Osłowski, S.; Ravi, V.; Reardon, D.; Toomey, L.; van Straten, W.; Wang, J. B.; Wen, L.; Zhu, X. J.

2015-08-01

Extreme scattering events (ESEs) in the interstellar medium (ISM) were first observed in regular flux measurements of compact extragalactic sources. They are characterized by a flux variation over a period of weeks, suggesting the passage of a “diverging plasma lens” across the line of sight (LOS). Modeling the refraction of such a lens indicates that the structure size must be of the order of AU and the electron density of the order of 10s of cm-3. Similar structures have been observed in measurements of pulsar intensity scintillation and group delay. Here we report observations of two ESEs, showing increases in both intensity scintillation and dispersion made with the Parkes Pulsar Timing Array. These allow us to make more complete models of the ESE, including an estimate of the “outer-scale” of the turbulence in the plasma lens. These observations clearly show that the ESE structure is fully turbulent on an AU scale. They provide some support for the idea that the structures are extended along the LOS, such as would be the case for a scattering shell. The dispersion measurements also show a variety of AU scale structures that would not be called ESEs, yet involve electron density variations typical of ESEs and likely have the same origin.

PULSAR OBSERVATIONS OF EXTREME SCATTERING EVENTS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Coles, W. A.; Kerr, M.; Shannon, R. M.

2015-08-01

Extreme scattering events (ESEs) in the interstellar medium (ISM) were first observed in regular flux measurements of compact extragalactic sources. They are characterized by a flux variation over a period of weeks, suggesting the passage of a “diverging plasma lens” across the line of sight (LOS). Modeling the refraction of such a lens indicates that the structure size must be of the order of AU and the electron density of the order of 10s of cm{sup −3}. Similar structures have been observed in measurements of pulsar intensity scintillation and group delay. Here we report observations of two ESEs, showing increasesmore » in both intensity scintillation and dispersion made with the Parkes Pulsar Timing Array. These allow us to make more complete models of the ESE, including an estimate of the “outer-scale” of the turbulence in the plasma lens. These observations clearly show that the ESE structure is fully turbulent on an AU scale. They provide some support for the idea that the structures are extended along the LOS, such as would be the case for a scattering shell. The dispersion measurements also show a variety of AU scale structures that would not be called ESEs, yet involve electron density variations typical of ESEs and likely have the same origin.« less

Stellar feedback as the origin of an extended molecular outflow in a starburst galaxy.

PubMed

Geach, J E; Hickox, R C; Diamond-Stanic, A M; Krips, M; Rudnick, G H; Tremonti, C A; Sell, P H; Coil, A L; Moustakas, J

2014-12-04

Recent observations have revealed that starburst galaxies can drive molecular gas outflows through stellar radiation pressure. Molecular gas is the phase of the interstellar medium from which stars form, so these outflows curtail stellar mass growth in galaxies. Previously known outflows, however, involve small fractions of the total molecular gas content and have typical scales of less than a kiloparsec. In at least some cases, input from active galactic nuclei is dynamically important, so pure stellar feedback (the momentum return into the interstellar medium) has been considered incapable of rapidly terminating star formation on galactic scales. Molecular gas has been detected outside the galactic plane of the archetypal starburst galaxy M82 (refs 4 and 5), but so far there has been no evidence that starbursts can propel substantial quantities of cold molecular gas to the same galactocentric radius (about 10 kiloparsecs) as the warmer gas that has been traced by metal ion absorbers in the circumgalactic medium. Here we report observations of molecular gas in a compact (effective radius 100 parsecs) massive starburst galaxy at redshift 0.7, which is known to drive a fast outflow of ionized gas. We find that 35 per cent of the total molecular gas extends approximately 10 kiloparsecs, and one-third of this extended gas has a velocity of up to 1,000 kilometres per second. The kinetic energy associated with this high-velocity component is consistent with the momentum flux available from stellar radiation pressure. This demonstrates that nuclear bursts of star formation are capable of ejecting large amounts of cold gas from the central regions of galaxies, thereby strongly affecting their evolution by truncating star formation and redistributing matter.

A Statistical Test of the Relationship between Galactic HI Structure and Small-scale Structure in the Cosmic Microwave Background

NASA Astrophysics Data System (ADS)

Verschuur, Gerrit L.

2014-06-01

The archive of IRIS, PLANCK and WMAP data available at the IRSA website of IPAC allows the apparent associations between galactic neutral hydrogen (HI) features and small-scale structure in WMAP and PLANCK data to be closely examined. In addition, HI new observations made with the Green Bank Telescope are used to perform a statistical test of putative associations. It is concluded that attention should be paid to the possibility that some of the small-scale structure found in WMAP and PLANCK data harbors the signature of a previously unrecognized source of high-frequency continuum emission in the Galaxy.

Plasmas in the outer heliosphere

NASA Technical Reports Server (NTRS)

Belcher, J. W.; Richardson, J. D.; Lazarus, A. J.; Gazis, P. R.; Barnes, A.

1995-01-01

We review the observed properties of the solar wind in the outer heliosphere, including observations from Voyager and the Pioneers, as well as from inner heliospheric probes as appropriate. These observations are crucial to modeling of the heliosphere and its interactions with the interstellar medium, since the wind ram pressure and its temporal variations are important in understanding the distance to the termination shock and heliopause and how those boundaries might vary in time. We focus on results since Solar Wind 7. Among the issues we will discuss are: (1) the time scales for and statistical properties of variations in the ram pressure in the outer heliosphere, and how those variations might affect the morphology of the heliospheric/interstellar medium interface; (2) the question of possible solar wind slowing in the outer heliosphere due to the pick-up of interstellar ions; (3) the issue of whether there is bulk heating of the solar wind associated either with interstellar ion pick-up or with continued heating due to stream-stream interactions; (4) evidence for latitudinal variations in solar wind properties; and (5) the 1.3 year periodicities apparent in the outer heliosphere, and the close correspondence with similar variations seen with inner heliospheric probes.

Coronal energy distribution and X-ray activity in the small scale magnetic field of the quiet sun

NASA Technical Reports Server (NTRS)

Habbal, S. R.

1992-01-01

The energy distribution in the small-scale magnetic field that pervades the solar surface, and its relationship to X-ray/coronal activity are discussed. The observed emission from the small scale structures, at temperatures characteristic of the chromosphere, transition region and corona, emanates from the boundaries of supergranular cells, within coronal bright points. This emission is characterized by a strong temporal and spatial variability with no definite pattern. The analysis of simultaneous, multiwavelength EUV observations shows that the spatial density of the enhanced as well as variable emission from the small scale structures exhibits a pronounced temperature dependence with significant maxima at 100,000 and 1,000,000 K. Within the limits of the spatial (1-5 arcsec) and temporal (1-5 min) resolution of data available at present, the observed variability in the small scale structure cannot account for the coroal heating of the quiet sun. The characteristics of their emission are more likely to be an indicator of the coronal heating mechanisms.

Interstellar Silicon Depletion and the Ultraviolet Extinction

NASA Astrophysics Data System (ADS)

Mishra, Ajay; Li, Aigen

2018-01-01

Spinning small silicate grains were recently invoked to account for the Galactic foreground anomalous microwave emission. These grains, if present, will absorb starlight in the far ultraviolet (UV). There is also renewed interest in attributing the enigmatic 2175 Å interstellar extinction bump to small silicates. To probe the role of silicon in the UV extinction, we explore the relations between the amount of silicon required to be locked up in silicates [Si/H]dust and the 2175 Å bump or the far-UV extinction rise, based on an analysis of the extinction curves along 46 Galactic sightlines for which the gas-phase silicon abundance [Si/H]gas is known. We derive [Si/H]dust either from [Si/H]ISM - [Si/H]gas or from the Kramers- Kronig relation which relates the wavelength-integrated extinction to the total dust volume, where [Si/H]ISM is the interstellar silicon reference abundance and taken to be that of proto-Sun or B stars. We also derive [Si/H]dust from fi�tting the observed extinction curves with a mixture of amorphous silicates and graphitic grains. We fi�nd that in all three cases [Si/H]dust shows no correlation with the 2175 Å bump, while the carbon depletion [C/H]dust tends to correlate with the 2175 Å bump. This supports carbon grains instead of silicates as the possible carrier of the 2175 Å bump. We also �find that neither [Si/H]dust nor [C/H]dust alone correlates with the far-UV extinction, suggesting that the far-UV extinction is a combined effect of small carbon grains and silicates.

Interstellar Silicon Depletion and the Ultraviolet Extinction

NASA Astrophysics Data System (ADS)

Mishra, Ajay; Li, Aigen

2017-12-01

Spinning small silicate grains were recently invoked to account for the Galactic foreground anomalous microwave emission. These grains, if present, will absorb starlight in the far-ultraviolet (UV). There is also renewed interest in attributing the enigmatic 2175 \\mathringA interstellar extinction bump to small silicates. To probe the role of silicon in the UV extinction, we explore the relations between the amount of silicon required to be locked up in silicates {[{Si}/{{H}}]}{dust} and the 2175 \\mathringA bump or the far-UV extinction rise, based on an analysis of the extinction curves along 46 Galactic sightlines for which the gas-phase silicon abundance {[{Si}/{{H}}]}{gas} is known. We derive {[{Si}/{{H}}]}{dust} either from {[{Si}/{{H}}]}{ISM}‑{[{Si}/{{H}}]}{gas} or from the Kramers–Kronig relation, which relates the wavelength-integrated extinction to the total dust volume, where {[{Si}/{{H}}]}{ISM} is the interstellar silicon reference abundance and taken to be that of proto-Sun or B stars. We also derive {[{Si}/{{H}}]}{dust} from fitting the observed extinction curves with a mixture of amorphous silicates and graphitic grains. We find that in all three cases {[{Si}/{{H}}]}{dust} shows no correlation with the 2175 \\mathringA bump, while the carbon depletion {[{{C}}/{{H}}]}{dust} tends to correlate with the 2175 \\mathringA bump. This supports carbon grains instead of silicates as the possible carriers of the 2175 \\mathringA bump. We also find that neither {[{Si}/{{H}}]}{dust} nor {[{{C}}/{{H}}]}{dust} alone correlates with the far-UV extinction, suggesting that the far-UV extinction is a combined effect of small carbon grains and silicates.

Small Body Exploration Technologies as Precursors for Interstellar Robotics

NASA Astrophysics Data System (ADS)

Noble, R. J.; Sykes, M. V.

The scientific activities undertaken to explore our Solar System will be very similar to those required someday at other stars. The systematic exploration of primitive small bodies throughout our Solar System requires new technologies for autonomous robotic spacecraft. These diverse celestial bodies contain clues to the early stages of the Solar System's evolution, as well as information about the origin and transport of water-rich and organic material, the essential building blocks for life. They will be among the first objects studied at distant star systems. The technologies developed to address small body and outer planet exploration will form much of the technical basis for designing interstellar robotic explorers. The Small Bodies Assessment Group, which reports to NASA, initiated a Technology Forum in 2011 that brought together scientists and technologists to discuss the needs and opportunities for small body robotic exploration in the Solar System. Presentations and discussions occurred in the areas of mission and spacecraft design, electric power, propulsion, avionics, communications, autonomous navigation, remote sensing and surface instruments, sampling, intelligent event recognition, and command and sequencing software. In this paper, the major technology themes from the Technology Forum are reviewed, and suggestions are made for developments that will have the largest impact on realizing autonomous robotic vehicles capable of exploring other star systems.

Small Body Exploration Technologies as Precursors for Interstellar Robotics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Noble, Robert; /SLAC; Sykes, Mark V.

The scientific activities undertaken to explore our Solar System will be the same as required someday at other stars. The systematic exploration of primitive small bodies throughout our Solar System requires new technologies for autonomous robotic spacecraft. These diverse celestial bodies contain clues to the early stages of the Solar System's evolution as well as information about the origin and transport of water-rich and organic material, the essential building blocks for life. They will be among the first objects studied at distant star systems. The technologies developed to address small body and outer planet exploration will form much of themore » technical basis for designing interstellar robotic explorers. The Small Bodies Assessment Group, which reports to NASA, initiated a Technology Forum in 2011 that brought together scientists and technologists to discuss the needs and opportunities for small body robotic exploration in the Solar System. Presentations and discussions occurred in the areas of mission and spacecraft design, electric power, propulsion, avionics, communications, autonomous navigation, remote sensing and surface instruments, sampling, intelligent event recognition, and command and sequencing software. In this paper, the major technology themes from the Technology Forum are reviewed, and suggestions are made for developments that will have the largest impact on realizing autonomous robotic vehicles capable of exploring other star systems.« less

Accurate determination of the fine-structure intervals in the 3P ground states of C-13 and C-12 by far-infrared laser magnetic resonance

NASA Technical Reports Server (NTRS)

Cooksy, A. L.; Saykally, R. J.; Brown, J. M.; Evenson, K. M.

1986-01-01

Accurate values are presented for the fine-structure intervals in the 3P ground state of neutral atomic C-12 and C-13 as obtained from laser magnetic resonance spectroscopy. The rigorous analysis of C-13 hyperfine structure, the measurement of resonant fields for C-12 transitions at several additional far-infrared laser frequencies, and the increased precision of the C-12 measurements, permit significant improvement in the evaluation of these energies relative to earlier work. These results will expedite the direct and precise measurement of these transitions in interstellar sources and should assist in the determination of the interstellar C-12/C-13 abundance ratio.

The cosmic-ray and gas content of the Cygnus region as measured in γ -rays by the Fermi Large Area Telescope

DOE PAGES

Ackermann, M.

2012-02-01

Context. The Cygnus region hosts a giant molecular-cloud complex that actively forms massive stars. Interactions of cosmic rays with interstellar gas and radiation fields make it shine at γ-ray energies. Several γ-ray pulsars and other energetic sources are seen in this direction. Aims. In this paper we analyze the γ-ray emission measured by the Fermi Large Area Telescope in the energy range from 100 MeV to 100 GeV in order to probe the gas and cosmic-ray content on the scale of the whole Cygnus complex. The γ-ray emission on the scale of the central massive stellar clusters and from individualmore » sources is addressed elsewhere. Methods. The signal from bright pulsars is greatly reduced by selecting photons in their off-pulse phase intervals. We compare the diffuse γ-ray emission with interstellar gas maps derived from radio/mm-wave lines and visual extinction data. A general model of the region, including other pulsars and γ-ray sources, is sought. Results. The integral Hi emissivity above 100 MeV averaged over the whole Cygnus complex amounts to [2.06 ± 0.11 (stat.) +0.15 -0.84 (syst.)] × 10 -26 photons s -1 sr -1 H-atom -1, where the systematic error is dominated by the uncertainty on the Hi opacity to calculate its column densities. The integral emissivity and its spectral energy distribution are both consistent within the systematics with LAT measurements in the interstellar space near the solar system. The average XCO = N(H2)/WCO ratio is found to be [1.68 ± 0.05 (stat.) +0.87 -0.10 (Hi opacity)] × 1020 molecules cm -2 (K km s -1) -1, consistent with other LAT measurements in the Local Arm. We detect significant γ-ray emission from dark neutral gas for a mass corresponding to ~ 40% of what is traced by CO. The total interstellar mass in the Cygnus complex inferred from its γ-ray emission amounts to 8 +5 -1 × 106M⊙ at a distance of 1.4 kpc. Conclusions. Despite the conspicuous star formation activity and high masses of the interstellar clouds, the cosmic-ray population in the Cygnus complex averaged over a few hundred parsecs is similar to that of the local interstellar space.« less

The Cosmic-Ray and Gas Content of the Cygnus Region as Measured in Gamma Rays by the Fermi Large Area Telescope

NASA Technical Reports Server (NTRS)

Ackermann, M.; Ajello, M.; Allafort, A.; Baldini, L.; Ballet, J.; Barbiellini, G.; Bastieri, D.; Belfiore, A.; Bellazzini, R.; Berenji, B.;

Self-Assembling Amphiphilic Molecules: A Possible Relationship Between Interstellar Chemistry and Meteoritic Organics

NASA Technical Reports Server (NTRS)

Sandford, Scott A.; Dworkin, Jason P.; Deamer, David W.; Allamandola, Louis J.; DeVincenzi, Donald (Technical Monitor)

2001-01-01

Interstellar gas and dust comprise the primary material from which the solar system formed. Evidence that some of this material was organic in nature and survived incorporation into the protosolar nebula is provided by the presence of deuterium-enriched organics in meteorites and interplanetary dust particles. Once the inner planets had sufficiently cooled, late accretionary infall of meteoroids and cosmic dust must have seeded them with some of these complex organic compounds. Delivery of such extraterrestrial compounds may have contributed to the organic inventory necessary for the origin of life. Interstellar ices, the building blocks of comets, tie up a large fraction of the biogenic elements available in molecular clouds. In our efforts to understand their synthesis, chemical composition, and physical properties, we report here that a complex mixture of molecules is produced by ultraviolet (UV) photolysis of realistic, interstellar ice analogs, and that some of the components have properties relevant to the origin of life, including the ability to self-assemble into vesicular structures.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lees, R. M.; Xu, Li-Hong; Appadoo, D. R. T.

New astronomical facilities such as HIFI on the Herschel Space Observatory, the SOFIA airborne IR telescope and the ALMA sub-mm telescope array will yield spectra from interstellar and protostellar sources with vastly increased sensitivity and frequency coverage. This creates the need for major enhancements to laboratory databases for the more prominent interstellar 'weed' species in order to model and account for their lines in observed spectra in the search for new and more exotic interstellar molecular 'flowers'. With its large-amplitude internal torsional motion, methanol has particularly rich spectra throughout the FIR and IR regions and, being very widely distributed throughoutmore » the galaxy, is perhaps the most notorious interstellar weed. Thus, we have recorded new spectra for a variety of methanol isotopic species on the high-resolution FTIR spectrometer on the CLS FIR beamline. The aim is to extend quantum number coverage of the data, improve our understanding of the energy level structure, and provide the astronomical community with better databases and models of the spectral patterns with greater predictive power for a range of astrophysical conditions.« less

New Insights Concerning the Local Interstellar medium

NASA Astrophysics Data System (ADS)

Linsky, Jeffrey L.; Redfield, Seth

2015-08-01

We have been analyzing HST high-resolution ultraviolet spectra of nearby stars to measure the radial velocities, turbulence, temperature, and depletions on warm diffuse interstellar gas within a few parsecs of the Sun. These data reveal a picture of many partially-ionized warm gas clouds, each with their own vector velocity and physical characteristics. This picture has been recently challenged by Gry and Jenkins (2014), who argue for a single nonrigid cloud surrounding the Sun. We present a test of these two very different morphological structure by checking how well each predicts the radial velocities in a new data set (Malamut et al. 2014) that was not available when both models were constructed. We find that the multicloud model (Redfield & Linsky 2008) provides a much better fit to the new data. We compare the new IBEX results for the temperature and velocity of inflowing He gas (McComas et al. 2015) with the properties of the Local Interstellar Cloud and the G cloud. We also show a preliminary three-dimensional model for the local interstellar medium.

Monte Carlo simulation to investigate the formation of molecular hydrogen and its deuterated forms

NASA Astrophysics Data System (ADS)

Sahu, Dipen; Das, Ankan; Majumdar, Liton; Chakrabarti, Sandip K.

2015-07-01

H2 is the most abundant interstellar species, and its deuterated forms (HD and D2) are also present in high abundance. The high abundance of these molecules could be explained by considering the chemistry that occurs on interstellar dust. Because of its simplicity, the rate equation method is widely used to study the formation of grain-surface species. However, because the recombination efficiency for the formation of any surface species is highly dependent on various physical and chemical parameters, the Monte Carlo method is best suited for addressing the randomness of the processes. We perform Monte Carlo simulations to study the formation of H2, HD and D2 on interstellar ice. The adsorption energies of surface species are the key inputs for the formation of any species on interstellar dusts, but the binding energies of deuterated species have yet to be determined with certainty. A zero-point energy correction exists between hydrogenated and deuterated species, which should be considered during modeling of the chemistry on interstellar dusts. Following some previous studies, we consider various sets of adsorption energies to investigate the formation of these species under diverse physical conditions. As expected, notable differences in these two approaches (rate equation method and Monte Carlo method) are observed for the production of these simple molecules on interstellar ice. We introduce two factors, namely, Sf and β , to explain these discrepancies: Sf is a scaling factor, which can be used to correlate the discrepancies between the rate equation and Monte Carlo methods, and β indicates the formation efficiency under various conditions. Higher values of β indicate a lower production efficiency. We observed that β increases with a decrease in the rate of accretion from the gas phase to the grain phase.

Blue Luminescence and Extended Red Emission: Possible Connections to the Diffuse Interstellar Bands

NASA Astrophysics Data System (ADS)

Witt, A. N.

2014-02-01

Blue luminescence (BL) and extended red emission (ERE) are observed as diffuse, optical-wavelength emissions in interstellar space, resulting from photoluminescence by ultraviolet(UV)-illuminated interstellar grains. Faintness and the challenge of separating the BL and ERE from the frequently much brighter dust-scattered continuum present major observational hurdles, which have permitted only slow progress in testing the numerous models that have been advanced to explain these two phenomena. Both the ERE, peaking near 680 nm (FWHM ~ 60 - 120 nm) and the BL, asymmetrically peaking at ~ 378 nm (FWHM ~ 45 nm), were first discovered in the Red Rectangle nebula. Subsequently, ERE and BL have been observed in other reflection nebulae, and in the case of the ERE, in carbon-rich planetary nebulae, H II regions, high-latitude cirrus clouds, the galactic diffuse ISM, and in external galaxies. BL exhibits a close spatial and intensity correlation with emission in the aromatic emission feature at 3.3 micron, most likely arising from small, neutral polycyclic aromatic hydrocarbon (PAH) molecules. The spectral characteristics of the BL also agree with those of fluorescence by PAH molecules with 13 to 19 carbon atoms. The BL phenomenon is thus most readily understood as the optical fluorescence of small, UV-excited aromatic molecules. The ERE, by contrast, though co-existent with mid-IR PAH emissions, does not correlate with emissions from either neutral or ionized PAHs. Instead, the spatial ERE morphology appears to be strictly governed by the density of far-UV (E >= 10.5 eV) photons, which are required for the ERE excitation. The most restrictive observational constraint for the ERE process is its exceptionally high quantum efficiency. If the ERE results from photo-excitation of a nano-particle carrier by photons with E >= 10.5 eV in a single-step process, the quantum efficiency exceeds 100%. Such a process, in which one to three low-energy optical photons may be emitted following a single far-UV excitation, is possible in highly isolated small clusters, e.g. small, dehydrogenated carbon clusters with about 20 to 28 carbon atoms. A possible connection between the ERE carriers and the carriers of DIBs may exist in that both are ubiquitous throughout the diffuse interstellar medium and both have an abundance of low-lying electronic levels with E <= 2.3 eV above the ground state.

Report

NASA Astrophysics Data System (ADS)

Kwok, Sun; Koo, Bon-Chul; Chu, You-Hua; Breitschwerdt, Dieter; De Gouveia Dal Pino, Elisabete; Yang, Ji; Tsuboi, Masato; Rozyczka, Michael; Juvela, Mika; Burton, Mike; Caselli, Paola; Lizano, Susana; Cabrit, Sylvie; Toth, Viktor

2016-04-01

Before 2012, Commission 34 was identical to Division VI. The organization and executive officers of the Division and the Commission were the same. At the 2012 General Assembly (GA) in Beijing, the IAU reformed the divisional structure and the previous Division VI which Commission 34 was under was combined with Div VII to form Division H: Interstellar Matter and Local Universe. Ewine van Dishoeck was named by the IAU executive committee as the President of the new Division. Since the Commission structure is to be reformed at the GA of 2015, Commission 34 retains its original name ``Interstellar Matter'' and joined Commissions 33 and 37 (formally under Division VII) as commissions under the new Division H.

Organics and Ices in the Outer Solar System: Connections to the Interstellar Medium

NASA Technical Reports Server (NTRS)

Pendleton, Y. J.; Cruikshank, D. P.

2017-01-01

The solar nebula, that aggregate of gas and dust that formed the birthplace of the Sun, planets and plethora of small bodies comprising the Solar System, originated in a molecular cloud that is thought to have spawned numerous additional stars, some with their own planets and attendant small bodies. The question of the chemical and physical reprocessing of the original interstellar materials in the solar nebula has challenged both theory and observations. The acquisition and analysis of samples of comet and asteroid solids, and a growing suite of in-situ and close-up analyses of relatively unaltered small Solar System bodies now adds critical new dimensions to the study of the origin and evolution of the early solar nebula. Better understanding the original composition of the material from which our solar nebula formed, and the processing that material experienced, will aid in formulations of chemistry that might occur in other solar systems. While we seek to understand the compositional history of planetary bodies in our own Solar System, we will inevitably learn more about the materials that comprise exoplanets and their surrounding systems.

Molecular Diagnostics of the Interstellar Medium and Star Forming Regions

NASA Astrophysics Data System (ADS)

Hartquist, T. W.; Dalgarno, A.

1996-03-01

Selected examples of the use of observationally inferred molecular level populations and chemical compositions in the diagnosis of interstellar sources and processes important in them (and in other diffuse astrophysical sources) are given. The sources considered include the interclump medium of a giant molecular cloud, dark cores which are the progenitors of star formation, material responding to recent star formation and which may form further stars, and stellar ejecta (including those of supernovae) about to merge with the interstellar medium. The measurement of the microwave background, mixing of material between different nuclear burning zones in evolved stars and turbulent boundary layers (which are present in and influence the structures and evolution of all diffuse astrophysical sources) are treated.

Relative distribution of cosmic rays and magnetic fields

NASA Astrophysics Data System (ADS)

Seta, Amit; Shukurov, Anvar; Wood, Toby S.; Bushby, Paul J.; Snodin, Andrew P.

2018-02-01

Synchrotron radiation from cosmic rays is a key observational probe of the galactic magnetic field. Interpreting synchrotron emission data requires knowledge of the cosmic ray number density, which is often assumed to be in energy equipartition (or otherwise tightly correlated) with the magnetic field energy. However, there is no compelling observational or theoretical reason to expect such a tight correlation to hold across all scales. We use test particle simulations, tracing the propagation of charged particles (protons) through a random magnetic field, to study the cosmic ray distribution at scales comparable to the correlation scale of the turbulent flow in the interstellar medium (≃100 pc in spiral galaxies). In these simulations, we find that there is no spatial correlation between the cosmic ray number density and the magnetic field energy density. In fact, their distributions are approximately statistically independent. We find that low-energy cosmic rays can become trapped between magnetic mirrors, whose location depends more on the structure of the field lines than on the field strength.

Developing Generic Image Search Strategies for Large Astronomical Data Sets and Archives using Convolutional Neural Networks and Transfer Learning

NASA Astrophysics Data System (ADS)

Peek, Joshua E. G.; Hargis, Jonathan R.; Jones, Craig K.

2018-01-01

Astronomical instruments produce petabytes of images every year, vastly more than can be inspected by a member of the astronomical community in search of a specific population of structures. Fortunately, the sky is mostly black and source extraction algorithms have been developed to provide searchable catalogs of unconfused sources like stars and galaxies. These tools often fail for studies of more diffuse structures like the interstellar medium and unresolved stellar structures in nearby galaxies, leaving astronomers interested in observations of photodissociation regions, stellar clusters, diffuse interstellar clouds without the crucial ability to search. In this work we present a new path forward for finding structures in large data sets similar to an input structure using convolutional neural networks, transfer learning, and machine learning clustering techniques. We show applications to archival data in the Mikulski Archive for Space Telescopes (MAST).

Dissipative structures of diffuse molecular gas. III. Small-scale intermittency of intense velocity-shears

NASA Astrophysics Data System (ADS)

Hily-Blant, P.; Falgarone, E.; Pety, J.

2008-04-01

Aims: We further characterize the structures tentatively identified on thermal and chemical grounds as the sites of dissipation of turbulence in molecular clouds (Papers I and II). Methods: Our study is based on two-point statistics of line centroid velocities (CV), computed from three large 12CO maps of two fields. We build the probability density functions (PDF) of the CO line centroid velocity increments (CVI) over lags varying by an order of magnitude. Structure functions of the line CV are computed up to the 6th order. We compare these statistical properties in two translucent parsec-scale fields embedded in different large-scale environments, one far from virial balance and the other virialized. We also address their scale dependence in the former, more turbulent, field. Results: The statistical properties of the line CV bear the three signatures of intermittency in a turbulent velocity field: (1) the non-Gaussian tails in the CVI PDF grow as the lag decreases, (2) the departure from Kolmogorov scaling of the high-order structure functions is more pronounced in the more turbulent field, (3) the positions contributing to the CVI PDF tails delineate narrow filamentary structures (thickness ~0.02 pc), uncorrelated to dense gas structures and spatially coherent with thicker ones (~0.18 pc) observed on larger scales. We show that the largest CVI trace sharp variations of the extreme CO linewings and that they actually capture properties of the underlying velocity field, uncontaminated by density fluctuations. The confrontation with theoretical predictions leads us to identify these small-scale filamentary structures with extrema of velocity-shears. We estimate that viscous dissipation at the 0.02 pc-scale in these structures is up to 10 times higher than average, consistent with their being associated with gas warmer than the bulk. Last, their average direction is parallel (or close) to that of the local magnetic field projection. Conclusions: Turbulence in these translucent fields exhibits the statistical and structural signatures of small-scale and inertial-range intermittency. The more turbulent field on the 30 pc-scale is also the more intermittent on small scales. The small-scale intermittent structures coincide with those formerly identified as sites of enhanced dissipation. They are organized into parsec-scale coherent structures, coupling a broad range of scales. Based on observations carried out with the IRAM-30 m telescope. IRAM is supported by INSU-CNRS/MPG/IGN.

Astronomical constraints on the cosmic evolution of the fine structure constant and possible quantum dimensions.

PubMed

Carilli, C L; Menten, K M; Stocke, J T; Perlman, E; Vermeulen, R; Briggs, F; de Bruyn , A G; Conway, J; Moore, C P

2000-12-25

We present measurements of absorption by the 21 cm hyperfine transition of neutral hydrogen toward radio sources at substantial look-back times. These data are used in combination with observations of rotational transitions of common interstellar molecules to set limits on the evolution of the fine structure constant: alpha/ alpha<3.5x10(-15) yr(-1), to a look-back time of 4.8 Gyr. In the context of string theory, the limit on the secular evolution of the scale factor of the compact dimensions, R, is &Rdot/ R<10(-15) yr(-1). Including terrestrial and other astronomical measurements places 2sigma limits on slow oscillations of R from the present to the epoch of cosmic nucleosynthesis, just seconds after the big bang, of DeltaR /R<10(-5).

Fabrication method for small-scale structures with non-planar features

DOEpatents

Burckel, David Bruce; Ten Eyck, Gregory A.

2016-09-20

The fabrication of small-scale structures is disclosed. A unit-cell of a small-scale structure with non-planar features is fabricated by forming a membrane on a suitable material. A pattern is formed in the membrane and a portion of the substrate underneath the membrane is removed to form a cavity. Resonators are then directionally deposited on the wall or sides of the cavity. The cavity may be rotated during deposition to form closed-loop resonators. The resonators may be non-planar. The unit-cells can be formed in a layer that includes an array of unit-cells.

Fabrication of small-scale structures with non-planar features

DOEpatents

Burckel, David B.; Ten Eyck, Gregory A.

2015-11-19

The fabrication of small-scale structures is disclosed. A unit-cell of a small-scale structure with non-planar features is fabricated by forming a membrane on a suitable material. A pattern is formed in the membrane and a portion of the substrate underneath the membrane is removed to form a cavity. Resonators are then directionally deposited on the wall or sides of the cavity. The cavity may be rotated during deposition to form closed-loop resonators. The resonators may be non-planar. The unit-cells can be formed in a layer that includes an array of unit-cells.

Laboratory investigation of the contribution of complex aromatic/aliphatic polycyclic hybrid molecular structures to interstellar ultraviolet extinction and infrared emission

NASA Technical Reports Server (NTRS)

Arnoult, K. M.; Wdowiak, T. J.; Beegle, L. W.

2000-01-01

We have demonstrated by experiment that, in an energetic environment, a simple polycyclic aromatic hydrocarbon (PAH) such as naphthalene will undergo chemical reactions that produce a wide array of more complex species (an aggregate). For a stellar wind of a highly evolved star (post-asymptotic giant branch [post-AGB]), this process would be in addition to what is expected from reactions occurring under thermodynamic equilibrium. A surprising result of that work was that produced in substantial abundance are hydrogenated forms that are hybrids of polycyclic aromatic and polycyclic alkanes. Infrared spectroscopy described here reveals a spectral character for these materials that has much in common with that observed for the constituents of circumstellar clouds of post-AGB stars. It can be demonstrated that a methylene (-CH2-) substructure, as in cycloalkanes, is the likely carrier of the 6.9 microns band emission of dust that has recently been formed around IRAS 22272+5433, NGC 7027, and CPD -56 8032. Ultraviolet spectroscopy previously done with a lower limit of 190 nm had revealed that this molecular aggregate can contribute to the interstellar extinction feature at 2175 angstroms. We have now extended our UV spectroscopy of these materials to 110 nm by a vacuum ultraviolet technique. That work, described here, reveals new spectral characteristics and describes how material newly formed during the late stages of stellar evolution could have produced an extinction feature claimed to exist at 1700 angstroms in the spectrum of HD 145502 and also how the newly formed hydrocarbon material would be transformed/aged in the general interstellar environment. The contribution of this molecular aggregate to the rise in interstellar extinction at wavelengths below 1500 angstroms is also examined. The panspectral measurements of the materials produced in the laboratory, using plasmas of H, He, N, and O to convert the simple PAH naphthalene to an aggregate of complex species, provide insight into possible molecular structure details of newly formed hydrocarbon-rich interstellar dust and its transformation into aged material that becomes resident in the interstellar medium. Specifically the presence of naphthalene-like and butadiene-like conjugated structures as chromophores for the 2175 angstroms ultraviolet extinction feature is indicated.

Collaborative Research: A Model of Partially Ionized Plasma Flows with Kinetic Treatment of Neutral Atoms and Nonthermal Ions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pogorelov, Nikolai; Zhang, Ming; Borovikov, Sergey

Interactions of flows of partially ionized, magnetized plasma are frequently accompanied by the presence of both thermal and non-thermal (pickup) ion components. Such interactions cannot be modeled using traditional MHD equations and require more advanced approaches to treat them. If a nonthermal component of ions is formed due to charge exchange and collisions between the thermal (core) ions and neutrals, it experiences the action of magnetic field, its distribution function is isotropized, and it soon acquires the velocity of the ambient plasma without being thermodynamically equilibrated. This situation, e. g., takes place in the outer heliosphere - the part ofmore » interstellar space beyond the solar system whose properties are determined by the solar wind interaction with the local interstellar medium. This is also possible in laboratory, at million degrees and above, when plasma is conducting electricity far too well, which makes Ohmic heating ineffective. To attain the target temperatures one needs additional heating eventually playing a dominant role. Among such sources is a so-called neutral particle beam heating. This is a wide-spread technique (Joint European Torus and International Thermonuclear Experimental Reactor experiments) based on the injection of powerful beams of neutral atoms into ohmically preheated plasma. In this project we have investigated the energy and density separation between the thermal and nonthermal components in the solar wind and interstellar plasmas. A new model has been developed in which we solve the ideal MHD equations for mixture of all ions and the kinetic Boltzmann equation to describe the transport of neutral atoms. As a separate capability, we can treat the flow of neutral atoms in a multi-component fashion, where neutral atoms born in each thermodynamically distinct regions are governed by the Euler gas dynamic equations. We also describe the behavior of pickup ions either kinetically, using the Fokker–Planck equation, or as a separate fluid. Our numerical simulations have demonstrated that pickup ions play a major role in the interaction of the solar wind and (partially ionized) interstellar medium plasmas. Our teams have investigated the stability of the surface (the heliopause) that separates the solar wind from the local interstellar medium, the transport of galactic cosmic rays, the properties of the heliotail flow, and modifications to the bow wave in front of the heliopause due to charge exchange between the neutral H atoms born in the solar wind and interstellar ions. Modeling results have been validated against observational data, such as obtained by the Interstellar Boundary Explorer (IBEX), and made it possible to shed light on the structure of energetic neutral atom maps created by this spacecraft.. We have also demonstrated that charge-exchange modulated heliosphere is a source of anisotropy of the multi-TeV cosmic ray flux observed in a number of Earth-bound air shower experiments. Newly developed codes are implemented within a Multi-Scale Fluid-Kinetic Simulation Suite (MS-FLUKSS), a publicly available code being developed by our team for over 12 years. MS-FLUKSS scales well up to 160,000 computing cores and has been ported on major supercomputers in the country. Efficient parallelization and data choreography in the continuum simulation modules are provided by Chombo, an adaptive mesh refinement framework managed by Phillip Colella’s team at LBNL. We have implemented in-house, hybrid (MPI+OpenMP) parallelization of the kinetic modules that solve the Boltzmann equation with a Monte Carlo method. Currently, the kinetic modules are being rewritten to take advantage of the modern CPU-GPU supercomputer architecture. The scope of the project allowed us to enhance plasma research and education in such broad, multidis- ciplinary field as physics of partially ionized plasma and its application to space physics and fusion science. Besides the impact on the modeling of complex physical systems, our approach to computational resource management for complex codes utilizing multiple algorithm technologies appears to be a major advance on current approaches. The development of sophisticated resource management will be essential for all future modeling efforts that incorporate a diversity of scales and physical processes. Our effort provided leadership in promoting computational science and plasma physics within the UAH and FIT campuses and, through the training of a broad spectrum of scientists and engineers, foster new technologies across the country.« less

Collaborative Research: A Model of Partially Ionized Plasma Flows with Kinetic Treatment of Neutral Atoms and Nonthermal Ions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pogorelov, Nikolai; Zhang, Ming

Interactions of flows of partially ionized, magnetized plasma are frequently accompanied by the presence of both thermal and non-thermal (pickup) ion components. Such interactions cannot be modeled using traditional MHD equations and require more advanced approaches to treat them. If a nonthermal component of ions is formed due to charge exchange and collisions between the thermal (core) ions and neutrals, it experiences the action of magnetic field, its distribution function is isotropized, and it soon acquires the velocity of the ambient plasma without being thermodynamically equilibrated. This situation, e. g., takes place in the outer heliosphere –- the part ofmore » interstellar space beyond the solar system whose properties are determined by the solar wind interaction with the local interstellar medium. This is also possible in laboratory, at million degrees and above, when plasma is conducting electricity far too well, which makes Ohmic heating ineffective. To attain the target temperatures one needs additional heating eventually playing a dominant role. Among such sources is a so-called neutral particle beam heating. This is a wide-spread technique (Joint European Torus and International Thermonuclear Experimental Reactor experiments) based on the injection of powerful beams of neutral atoms into ohmically preheated plasma. In this project we have investigated the energy and density separation between the thermal and nonthermal components in the solar wind and interstellar plasmas. A new model has been developed in which we solve the ideal MHD equations for mixture of all ions and the kinetic Boltzmann equation to describe the transport of neutral atoms. As a separate capability, we can treat the flow of neutral atoms in a multi-component fashion, where neutral atoms born in each thermodynamically distinct region are governed by the Euler gas dynamic equations. We also describe the behavior of pickup ions either kinetically, using the Fokker--Planck equation, or as a separate fluid. Our numerical simulations have demonstrated that pickup ions play a major role in the interaction of the solar wind and (partially ionized) interstellar medium plasmas. Our teams have investigated the stability of the surface (the heliopause) that separates the solar wind from the local interstellar medium, the transport of galactic cosmic rays, the properties of the heliotail flow, and modifications to the bow wave in front of the heliopause due to charge exchange between the neutral H atoms born in the solar wind and interstellar ions. Modeling results have been validated against observational data, such as obtained by the Interstellar Boundary Explorer (IBEX), and made it possible to shed light on the structure of energetic neutral atom maps created by this spacecraft.. We have also demonstrated that charge-exchange modulated heliosphere is a source of anisotropy of the multi-TeV cosmic ray flux observed in a number of Earth-bound air shower experiments. Newly developed codes are implemented within a Multi-Scale Fluid-Kinetic Simulation Suite (MS-FLUKSS), a publicly available code being developed by our team for over 12 years. MS-FLUKSS scales well up to 160,000 computing cores and has been ported on major supercomputers in the country. Efficient parallelization and data choreography in the continuum simulation modules are provided by Chombo, an adaptive mesh refinement framework managed by Phillip Colella's team at LBNL. We have implemented in-house, hybrid (MPI+OpenMP) parallelization of the kinetic modules that solve the Boltzmann equation with a Monte Carlo method. Currently, the kinetic modules are being rewritten to take advantage of the modern CPU-GPU supercomputer architecture. The scope of the project allowed us to enhance plasma research and education in such broad, multidisciplinary field as physics of partially ionized plasma and its application to space physics and fusion science. Besides the impact on the modeling of complex physical systems, our approach to computational resource management for complex codes utilizing multiple algorithm technologies appears to be a major advance on current approaches. The development of sophisticated resource management will be essential for all future modeling efforts that incorporate a diversity of scales and physical processes. Our effort provided leadership in promoting computational science and plasma physics within the UAH and FIT campuses and, through the training of a broad spectrum of scientists and engineers, fostering new technologies across the country.« less

Modeling the processing of interstellar ices by energetic particles

NASA Astrophysics Data System (ADS)

Kalvāns, J.; Shmeld, I.

2013-06-01

Context. Interstellar ice is the main form of metal species in dark molecular clouds. Experiments and observations have shown that the ice is significantly processed after the freeze-out of molecules onto grains. The processing is caused by cosmic-ray particles and cosmic-ray-induced UV photons. These transformations are included in current astrochemical models only to a very limited degree. Aims: We aim to establish a model of the "cold" chemistry in interstellar ices and to evaluate its general impact on the composition of interstellar ices. Methods: The ice was treated as consisting of two layers - the surface and the mantle (or subsurface) layer. Subsurface chemical processes are described with photodissociation of ice species and binary reactions on the surfaces of cavities inside the mantle. Hydrogen atoms and molecules can diffuse between the layers. We also included deuterium chemistry. Results: The modeling results show that the content of chemically bound H is reduced in subsurface molecules by about 30% on average. This promotes the formation of more hydrogen-poor species in the ice. The enrichment of ice molecules with deuterium is significantly reduced by the subsurface processes. On average, it follows the gas-phase atomic D/H abundance ratio, with a delay. The delay produced by the model is on the order of several Myr. Conclusions: The processing of ice may place new constraints on the production of deuterated species on grains. In a mantle with a two-layer structure the upper layer (CO) should be processed substantially more intensively than the lower layer (H2O). Chemical explosions in interstellar ice might not be an important process. They destroy the structure of the mantle, which forms over long timescales. Besides, ices may lack the high radical content needed for the explosions.

Constraints on cosmic silicates

NASA Astrophysics Data System (ADS)

Ossenkopf, V.; Henning, Th.; Mathis, J. S.

1992-08-01

Observational determinations of opacities of circumstellar silicates, relative to the peak value near 10 microns, are used to estimate the optical constants n and k, the real and imaginary parts of the index of refraction. Circumstellar dust is modified by processing within the interstellar medium. This leads to higher band strengths and a somewhat larger ratio of the opacities at the 18 and 10-micron peaks, compared with circumstellar silicates. By using an effective-medium theory, we calculate the effects of small spherical inclusions of various materials (various oxides, sulfides, carbides, amorphous carbon, and metallic iron) upon silicate opacities. Some of these can increase the absorption coefficient k in the 2-8 micron region appreciably, as is needed to reconcile laboratory silicate opacities with observations of both the interstellar medium and envelopes around late-type stars. We give tables of two sets of optical constants for warm oxygen-deficient and cool oxygen-rich silicates, representative for circumstellar and interstellar silicates. The required opacity in the 2-8 micron region is provided by iron and magnetite.

Coherent array of branched filamentary scales along the wing margin of a small moth

NASA Astrophysics Data System (ADS)

Yoshida, Akihiro; Tejima, Shin; Sakuma, Masayuki; Sakamaki, Yositaka; Kodama, Ryuji

2017-04-01

In butterflies and moths, the wing margins are fringed with specialized scales that are typically longer than common scales. In the hindwings of some small moths, the posterior margins are fringed with particularly long filamentary scales. Despite the small size of these moth wings, these scales are much longer than those of large moths and butterflies. In the current study, photography of the tethered flight of a small moth, Phthorimaea operculella, revealed a wide array composed of a large number of long filamentary scales. This array did not become disheveled in flight, maintaining a coherent sheet-like structure during wingbeat. Examination of the morphology of individual scales revealed that each filamentary scale consists of a proximal stalk and distal branches. Moreover, not only long scales but also shorter scales of various lengths were found to coexist in each small section of the wing margin. Scale branches were ubiquitously and densely distributed within the scale array to form a mesh-like architecture similar to a nonwoven fabric. We propose that possible mechanical interactions among branched filamentary scales, mediated by these branches, may contribute to maintaining a coherent sheet-like structure of the scale array during wingbeat.

The Challenges of Low-Frequency Radio Polarimetry: Lessons from the Murchison Widefield Array

NASA Astrophysics Data System (ADS)

Lenc, E.; Anderson, C. S.; Barry, N.; Bowman, J. D.; Cairns, I. H.; Farnes, J. S.; Gaensler, B. M.; Heald, G.; Johnston-Hollitt, M.; Kaplan, D. L.; Lynch, C. R.; McCauley, P. I.; Mitchell, D. A.; Morgan, J.; Morales, M. F.; Murphy, Tara; Offringa, A. R.; Ord, S. M.; Pindor, B.; Riseley, C.; Sadler, E. M.; Sobey, C.; Sokolowski, M.; Sullivan, I. S.; O'Sullivan, S. P.; Sun, X. H.; Tremblay, S. E.; Trott, C. M.; Wayth, R. B.

2017-09-01

We present techniques developed to calibrate and correct Murchison Widefield Array low-frequency (72-300 MHz) radio observations for polarimetry. The extremely wide field-of-view, excellent instantaneous (u, v)-coverage and sensitivity to degree-scale structure that the Murchison Widefield Array provides enable instrumental calibration, removal of instrumental artefacts, and correction for ionospheric Faraday rotation through imaging techniques. With the demonstrated polarimetric capabilities of the Murchison Widefield Array, we discuss future directions for polarimetric science at low frequencies to answer outstanding questions relating to polarised source counts, source depolarisation, pulsar science, low-mass stars, exoplanets, the nature of the interstellar and intergalactic media, and the solar environment.

The volume- and surface-binding energies of ice systems containing CO, CO2, and H2O

NASA Technical Reports Server (NTRS)

Sandford, Scott A.; Allamandola, Louis J.

1990-01-01

Laboratory-measured, temperature-dependent sticking efficiencies are presently used to derive the surface-binding energies of CO and CO2 on H2O-rich ices, with a view to determining the condensation and vaporization properties of these systems as well as to the measured energies' implications for both cometary behavior and the evolution of interstellar ices. The molecular volume and the surface binding energies are not found to be necessarily related on the basis of simple nearest-neighbor scaling in surface and bulk sites; this may be due to the physical constraints associated with matrix structure-associated physical constraints, which sometimes dominate the volume-binding energies.

Investigations of the Formation of Carbon Grains in Circumstellar Outflows

NASA Technical Reports Server (NTRS)

Contreras, Cesar; Salama, Farid

2013-01-01

The study of formation and destruction processes of cosmic dust is essential to understand and to quantify the budget of extraterrestrial organic molecules. Although dust with all its components plays an important role in the evolution of interstellar chemistry and in the formation of organic molecules, little is known on the formation and destruction processes of carbonaceous dust. PAHs are important chemical building blocks of interstellar dust. They are detected in interplanetary dust particles and in meteoritic samples. Additionally, observational, laboratory, and theoretical studies have shown that PAHs, in their neutral and ionized forms, are an important, ubiquitous component of the interstellar medium. Also, the formation of PAHs from smaller molecules has not been extensively studied. Therefore, it is imperative that laboratory experiments be conducted to study the dynamic processes of carbon grain formation from PAH precursors. Studies of interstellar dust analogs formed from a variety of PAH and hydrocarbon precursors as well as species that include the atoms O, N, and S, have recently been performed in our laboratory under conditions that simulate interstellar and circumstellar environments. The species formed in the pulsed discharge nozzle (PDN) plasma source are detected and characterized with a high-sensitivity cavity ringdown spectrometer (CRDS) coupled to a Reflectron time-of-flight mass spectrometer (ReTOF-MS), thus providing both spectroscopic and ion mass information in-situ. We report the first set of measurements obtained in these experiments and identify the species present in the experiments and the ions that are formed in the plasma process. From these unique measurements, we derive information on the size and the structure of interstellar dust grain particles, the growth and the destruction processes of interstellar dust and the resulting budget of extraterrestrial organic molecules.

Time Dependent Density Functional Theory Calculations of Large Compact PAH Cations: Implications for the Diffuse Interstellar Bands

NASA Technical Reports Server (NTRS)

Weisman, Jennifer L.; Lee, Timothy J.; Salama, Farid; Gordon-Head, Martin; Kwak, Dochan (Technical Monitor)

2002-01-01

We investigate the electronic absorption spectra of several maximally pericondensed polycyclic aromatic hydrocarbon radical cations with time dependent density functional theory calculations. We find interesting trends in the vertical excitation energies and oscillator strengths for this series containing pyrene through circumcoronene, the largest species containing more than 50 carbon atoms. We discuss the implications of these new results for the size and structure distribution of the diffuse interstellar band carriers.

IBEX-lo Sky Maps of Secondary Interstellar Neutrals Helium and Oxygen

NASA Astrophysics Data System (ADS)

Kucharek, H.; Isenberg, P. A.; Jeewoo, P.; Kubiak, M. A.; Bzowski, M.

2017-12-01

There are several populations of heliospheric energetic neutral atoms (ENAs) generated at the various heliospheric interfaces, the inner heliosheath, outer heliosheath (OHS), and the termination shock (TS). Depending on where and how these ENAs are generated, they belong to different energy regimes. While interstellar neutral (ISN) particles flow through the heliospheric boundary is mostly unimpeded, a substantial fraction of ISN H and O is filtered through charge exchange with ambient plasma ions before reaching the TS. Secondary ISN atoms are generated by the charge exchange reaction between primary ISN atoms and interstellar ions in the outer heliosheath, forming walls of H and O in front of the heliopause (HP). The flowing interstellar plasma encounters the heliopause as an obstacle, which deflects the flow. Thus, secondary neutrals measured at 1 AU carry information about the deflected interstellar plasma and the shape of the heliopause that causes the deflection. Due to very different magnitudes of charge exchange cross sections, the main source of the secondary He is charge exchange with the OHS He+, while that of the secondary O is the charge exchange between interstellar O+ and the OHS H. Therefore, the oxygen results are drastically different from those of helium. Interstellar O+ ions behave in principle like the He+ particles with an over-density due to the plasma deceleration. The high density decelerated oxygen ions just upwind of the heliopause encounter an over-density in neutral hydrogen, the hydrogen wall, allowing frequent charge exchange that produce slow neutral oxygen atoms forming the oxygen wall. Thus, the distribution in the sky maps of secondary He and O carries information on the shape as well as the structures in front of it. To investigate the secondary component of the interstellar neutral in detail we have distinguish between the two secondary component's. We engaged theory and simulations for the primary and secondary components to determine differences of between measurements and model predicted data.

New Homogeneous Standards by Atomic Layer Deposition for Synchrotron X-ray Fluorescence and Absorption Spectroscopies.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Butterworth, A.L.; Becker, N.; Gainsforth, Z.

2012-03-13

Quantification of synchrotron XRF analyses is typically done through comparisons with measurements on the NIST SRM 1832/1833 thin film standards. Unfortunately, these standards are inhomogeneous on small scales at the tens of percent level. We are synthesizing new homogeneous multilayer standards using the Atomic Layer Deposition technique and characterizing them using multiple analytical methods, including ellipsometry, Rutherford Back Scattering at Evans Analytical, Synchrotron X-ray Fluorescence (SXRF) at Advanced Photon Source (APS) Beamline 13-ID, Synchrotron X-ray Absorption Spectroscopy (XAS) at Advanced Light Source (ALS) Beamlines 11.0.2 and 5.3.2.1 and by electron microscopy techniques. Our motivation for developing much-needed cross-calibration of synchrotronmore » techniques is borne from coordinated analyses of particles captured in the aerogel of the NASA Stardust Interstellar Dust Collector (SIDC). The Stardust Interstellar Dust Preliminary Examination (ISPE) team have characterized three sub-nanogram, {approx}1{micro}m-sized fragments considered as candidates to be the first contemporary interstellar dust ever collected, based on their chemistries and trajectories. The candidates were analyzed in small wedges of aerogel in which they were extracted from the larger collector, using high sensitivity, high spatial resolution >3 keV synchrotron x-ray fluorescence spectroscopy (SXRF) and <2 keV synchrotron x-ray transmission microscopy (STXM) during Stardust ISPE. The ISPE synchrotron techniques have complementary capabilities. Hard X-ray SXRF is sensitive to sub-fg mass of elements Z {ge} 20 (calcium) and has a spatial resolution as low as 90nm. X-ray Diffraction data were collected simultaneously with SXRF data. Soft X-ray STXM at ALS beamline 11.0.2 can detect fg-mass of most elements, including cosmochemically important oxygen, magnesium, aluminum and silicon, which are invisible to SXRF in this application. ALS beamline 11.0.2 has spatial resolution better than 25 nm. Limiting factors for Stardust STXM analyses were self-imposed limits of photon dose due to radiation damage concerns, and significant attenuation of <1500 eV X-rays by {approx}80{micro}m thick, {approx}25 mg/cm{sup 3} density silica aerogel capture medium. In practice, the ISPE team characterized the major, light elements using STXM (O, Mg, Al, Si) and the heavier minor and trace elements using SXRF. The two data sets overlapped only with minor Fe and Ni ({approx}1% mass abundance), providing few quantitative cross-checks. New improved standards for cross calibration are essential for consortium-based analyses of Stardust interstellar and cometary particles, IDPs. Indeed, they have far reaching application across the whole synchrotron-based analytical community. We have synthesized three ALD multilayers simultaneously on silicon nitride membranes and silicon and characterized them using RBS (on Si), XRF (on Si{sub 3}N{sub 4}) and STXM/XAS (holey Si{sub 3}N{sub 4}). The systems we have started to work with are Al-Zn-Fe and Y-Mg-Er. We have found these ALD multi-layers to be uniform at {micro}m- to nm scales, and have found excellent consistency between four analytical techniques so far. The ALD films can also be used as a standard for e-beam instruments, eg., TEM EELS or EDX. After some early issues with the consistency of coatings to the back-side of the membrane windows, we are confident to be able to show multi-analytical agreement to within 10%. As the precision improves, we can use the new standards to verify or improve the tabulated cross-sections.« less

First-principles study of the formation of glycine-producing radicals from common interstellar species

NASA Astrophysics Data System (ADS)

Sato, Akimasa; Kitazawa, Yuya; Ochi, Toshiro; Shoji, Mitsuo; Komatsu, Yu; Kayanuma, Megumi; Aikawa, Yuri; Umemura, Masayuki; Shigeta, Yasuteru

2018-03-01

Glycine, the simplest amino acid, has been intensively searched for in molecular clouds, and the comprehensive clarification of the formation path of interstellar glycine is now imperative. Among all the possible glycine formation pathways, we focused on the radical pathways revealed by Garrod (2013). In the present study, we have precisely investigated all the chemical reaction steps related to the glycine formation processes based on state-of-the-art density functional theory (DFT) calculations. We found that two reaction pathways require small activation barriers (ΔE‡ ≤ 7.75 kJ mol-1), which demonstrates the possibility of glycine formation even at low temperatures in interstellar space if the radical species are generated. The origin of carbon and nitrogen in the glycine backbone and their combination patterns are further discussed in relation to the formation mechanisms. According to the clarification of the atomic correspondence between glycine and its potential parental molecules, it is shown that the nitrogen and two carbons in the glycine can originate in three common interstellar molecules, methanol, hydrogen cyanide, and ammonia, and that the source molecules of glycine can be described by any of their combinations. The glycine formation processes can be categorized into six patterns. Finally, we discussed two other glycine formation pathways expected from the present DFT calculation results.

The high brightness temperature of B0529+483 revealed by RadioAstron and implications for interstellar scattering

NASA Astrophysics Data System (ADS)

Pilipenko, S. V.; Kovalev, Y. Y.; Andrianov, A. S.; Bach, U.; Buttaccio, S.; Cassaro, P.; Cimò, G.; Edwards, P. G.; Gawroński, M. P.; Gurvits, L. I.; Hovatta, T.; Jauncey, D. L.; Johnson, M. D.; Kovalev, Yu A.; Kutkin, A. M.; Lisakov, M. M.; Melnikov, A. E.; Orlati, A.; Rudnitskiy, A. G.; Sokolovsky, K. V.; Stanghellini, C.; de Vicente, P.; Voitsik, P. A.; Wolak, P.; Zhekanis, G. V.

2018-03-01

The high brightness temperatures, Tb ≳ 1013 K, detected in several active galactic nuclei by RadioAstron space VLBI observations challenge theoretical limits. Refractive scattering by the interstellar medium may affect such measurements. We quantify the scattering properties and the sub-mas scale source parameters for the quasar B0529+483. Using RadioAstron correlated flux density measurements at 1.7, 4.8, and 22 GHz on projected baselines up to 240 000 km we find two characteristic angular scales in the quasar core, about 100 and 10 μas. Some indications of scattering substructure are found. Very high brightness temperatures, Tb ≥ 1013 K, are estimated at 4.8 and 22 GHz even taking into account the refractive scattering. Our findings suggest a clear dominance of the particle energy density over the magnetic field energy density in the core of this quasar.

The Contribution of Galactic Free-Free Emission to Anistropies in the Cosmic Microwave Background Found by the Saskatoon Experiment

NASA Astrophysics Data System (ADS)

Simonetti, John H.; Dennison, Brian; Topasna, Gregory A.

1996-02-01

We made a sensitive, wide-field H alpha image of the north celestial polar region. Using this image, we constrain the contribution of irregularities in interstellar free-free emission to the degree-scale anisotropies in the cosmic microwave background detected in recent observations at Saskatoon by the Princeton group. The analysis of the H alpha image mimics the Saskatoon data analysis: the resulting signal is the strength of irregularities sampled with the Saskatoon beam (i.e., degree-scale) along the 85 deg declination circle. We found no such irregularities that could be attributed to H alpha emission. The implied upper bound on the rms variation in free-free brightness temperature is less than 4.6 mu K at 27.5 GHz. The observed cosmic microwave background anisotropies are much larger. Therefore, the contribution of irregularities in interstellar free-free emission to the observed anisotropies is negligible.

New ALMA Images of the HD 32297 and HD 61005 Debris Disks

NASA Astrophysics Data System (ADS)

MacGregor, Meredith Ann; Weinberger, Alycia; Wilner, David; Hughes, A. Meredith; debes, John Henry; Redfield, Seth; Donaldson, Jessica; Nesvold, Erika; Schneider, Glenn; Currie, Thayne; Roberge, Aki; Rodriguez, David

2018-01-01

HD 61005 (G-type star, “The Moth") and HD 32297 (A-type star) host two of the most iconic debris disks. Scattered light images show that both disks are nearly edge-on with dramatic swept-back wings of dust. Previous studies have proposed a range of mechanisms to explain this distinctive morphology including interactions with the interstellar medium, secular perturbations of grains by low-density, neutral interstellar gas, and gravitational interactions with an inclined, eccentric companion. We present new observations from the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.3 mm that provide the highest resolution images at millimeter wavelengths to date of both systems. Observations at millimeter wavelengths are especially critical to our understanding of the physical mechanisms shaping the structure of these disks, since the large grains that dominate emission at these wavelengths are less affected by stellar radiation and winds and more reliably trace the underlying planetesimal distribution. We fit models directly to the observed visibilities within a Markov Chain Monte Carlo (MCMC) framework to characterize the continuum emission and place constraints on the structure of these unique debris disks. Our new ALMA images reveal that despite differences in spectral type, both systems are best described by a two-component structure with (1) a parent body belt, and (2) an outer halo aligned with the scattered light disk. Such halos have typically been assumed to be composed of small grains visible in scattered light, so these images are some of the first observational evidence that larger grains may also populate extended halos. In addition, we detect significant 12CO gas emission from HD 32297, and determine a robust upper limit for HD 61005.

Relative f-values from interstellar absorption lines: advantages and pitfalls

NASA Astrophysics Data System (ADS)

Jenkins, Edward B.

2009-05-01

Interstellar absorption features seen in the ultraviolet and visible spectra of stars provide opportunities for comparing the strengths of different transitions out of the ground electronic states of atoms, ions and simple molecules. In principle, such measurements are straightforward since the radiative transfer is manifested as a simple exponential absorption law at any given radial velocity. Complications arise when the velocity structures of the lines are not completely resolved, or when the lines are either very strongly saturated or too weak to observe. Dynamic range limitations can compromise the comparisons of two transitions that have very different absorption f-values, but they can be mitigated if there are examples with very different column densities and transitions of intermediate strength that can help to bridge the large gap in line strengths. Attempts to unravel the effects of saturation include the use of a curve of growth when only equivalent widths are available, or the measurements of the 'apparent optical depth' when the line is mostly resolved by the instrument. Unfortunately, the application of the curve of growth for one constituent to that of another can sometimes create systematic errors, since the two may have different velocity structures. Likewise, unresolved fine velocity structures in features that have large optical depths can make the apparent optical depths misrepresent the smoothed versions of the true optical depths. One method to compare the strength of a very weak line to that of a very strong one is to measure the total absorption of the former and compare it with the strength of the damping wings of the latter. However in many circumstances, small amounts of gas at velocities well displaced from the line center can masquerade as damping wings. For this reason, it is important to check that these wings have the proper shape.

Turbulence in nature and in the laboratory

PubMed Central

Warhaft, Z.

2002-01-01

Fluid turbulence has attracted the attention of physicists, mathematicians, and engineers for over 100 years, yet it remains an unsolved problem. The reasons for the difficulties are outlined and recent advances in describing its small-scale statistical structure are described. Contrary to traditional notions, new experimental evidence indicates that the small scales are anisotropic, reflecting the overall character of the flow. The consequences of this finding with regard to the long-held postulate of the universality of the small-scale turbulence structure are discussed. PMID:11875199

The Inventory of Interstellar Materials Available for the Formation of the Solar System

NASA Technical Reports Server (NTRS)

Sandford, Scott A.; Witteborn, Fred C. (Technical Monitor)

1996-01-01

Dr. Derek Sears, the editor of the journal Meteoritics and Planetary Science, has established a policy of having each issue of the journal contain an invited review of an area that he deems to be of special cur-rent importance. Typically 20 to 25 pages of the beginning of the journal are devoted to each review. He has asked me to prepare such a review summarizing what we know about the composition and structure of interstellar materials. The attached paper is the result. This is a good time for such a review since tremendous progress has been made in the field of interstellar dust in recent years through the use of telescopic observations, theoretical studies, laboratory studies of analogs, and the study of actual interstellar samples found in meteorites. It is increasing clear that the interstellar medium (ISM) contains an enormous diversity of materials created by a wide range of chemical and physical processes. This understanding is a far cry from the picture of interstellar materials held as recently as two decades ago, a picture which incorporated only a few generic types of grains and few molecules. In the paper I review our current knowledge of the more abundant materials thought to exist in the ISM. The review concentrates on matter in interstellar dense molecular clouds since it is the materials in these environments from which new stars and planetary systems are formed, although materials in circumstellar environments and in the diffuse ISM are also discussed. The paper focuses largely on solid materials since they contain a major fraction of the heavier elements in clouds and because solids are most likely to survive incorporation into new planetary systems in identifiable form. The paper concludes with discussion of some of the implications resulting from the identification of these interstellar materials. I also present some new thoughts, the most intriguing being that meteoritic 'microdiamonds' may be the same material that modelers of the interstellar extinction call graphite! If my suggestion is correct it will help resolve the dilemma associated with the current over-subscription of cosmic C and should lead to some new approaches to both extinction models and the study of meteoritic interstellar microdiamonds.

Examination of Cross-Scale Coupling During Auroral Events using RENU2 and ISINGLASS Sounding Rocket Data.

NASA Astrophysics Data System (ADS)

Kenward, D. R.; Lessard, M.; Lynch, K. A.; Hysell, D. L.; Hampton, D. L.; Michell, R.; Samara, M.; Varney, R. H.; Oksavik, K.; Clausen, L. B. N.; Hecht, J. H.; Clemmons, J. H.; Fritz, B.

2017-12-01

The RENU2 sounding rocket (launched from Andoya rocket range on December 13th, 2015) observed Poleward Moving Auroral Forms within the dayside cusp. The ISINGLASS rockets (launched from Poker Flat rocket range on February 22, 2017 and March 2, 2017) both observed aurora during a substorm event. Despite observing very different events, both campaigns witnessed a high degree of small scale structuring within the larger auroral boundary, including Alfvenic signatures. These observations suggest a method of coupling large-scale energy input to fine scale structures within aurorae. During RENU2, small (sub-km) scale drivers persist for long (10s of minutes) time scales and result in large scale ionospheric (thermal electron) and thermospheric response (neutral upwelling). ISINGLASS observations show small scale drivers, but with short (minute) time scales, with ionospheric response characterized by the flight's thermal electron instrument (ERPA). The comparison of the two flights provides an excellent opportunity to examine ionospheric and thermospheric response to small scale drivers over different integration times.

Open Questions, New Instrumentation, and Challenges for Heliospheric Physics beyond 2020

NASA Astrophysics Data System (ADS)

Desai, Mihir; Allegrini, Frederic

The last decade has seen tremendous breakthroughs in our knowledge of the outer edges of the heliosphere and the interaction between the Sun and its local galactic neighborhood. These advances include the crossing of the termination shock and perhaps the heliopause by Voyager 1 and global imaging of energetic neutral atom (ENA) emission from the outer heliosphere by IBEX and Cassini. IBEX discovered a narrow “ribbon” of ENA emissions encircling the heliosphere, and provided direct measurements of interstellar neutral atoms that point to the absence of a bow shock beyond the heliopause. The big picture provided by IBEX, complemented by Voyager observations, shows that the asymmetry of the heliosphere is shaped by the surrounding interstellar magnetic field and that the physical processes that control the interaction exist on relatively small spatial and temporal scales (months) that are not currently measured. Additionally, in-situ observations from ACE, Wind, SoHO, SAMPEX, and STEREO have contributed dramatically to our understanding of solar energetic particle (SEP) events, of the importance of suprathermal ions for efficient energization, of the sources and evolution of solar wind, interplanetary magnetic field, corona mass ejections (CMEs), and SEPs that impact geospace and the heliosphere. These phenomena are controlled by myriad complex and poorly understood physical effects that must be unraveled to develop a complete picture of particle acceleration and transport and of the causes and impacts of interplanetary disturbances on geospace and the heliosphere. In this talk I will summarize our current state of knowledge in heliospheric physics, identify key questions that will be addressed by upcoming missions like Solar Probe Plus and Solar Orbiter, and then discuss a new set of challenges that need to be met in order to obtain a complete understanding of the solar and interplanetary drivers of Space Weather and SEPs, and to discover how our heliosphere interacts with the nearby interstellar and galactic environments.

Fifteen Years of Laboratory Astrophysics at Ames

NASA Technical Reports Server (NTRS)

Allamandola, L. J.; Sandford, S. A.; Salama, F.; Hudgins, D. M.; Bernstein, M.; Goorvitch, David (Technical Monitor)

1998-01-01

Tremendous strides have been made in our understanding of interstellar material over the past fifteen years thanks to significant, parallel developments in two closely related areas: observational astronomy and laboratory astrophysics. Fifteen years ago the composition of interstellar dust was largely guessed at, the concept of ices in dense molecular clouds ignored, and the notion of large, abundant, gas phase, carbon-rich molecules widespread throughout the interstellar medium (ISM) considered impossible. Today the composition of dust in the diffuse ISM is reasonably well constrained to cold refractory materials comprised of amorphous and crystalline silicates mixed with an amorphous carbonaceous material containing aromatic structural units and short, branched aliphatic chains. In the dense ISM, these cold dust particles are coated with mixed-molecular ices whose compositions are very well known. Lastly, the signature of carbon-rich polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by early interstellar chemistry standards, is widespread throughout the ISM. This great progress has only been made possible by the close collaboration of laboratory experimentalists with observers and theoreticians, all with the goal of applying their skills to astrophysical problems of direct interest to NASA programs. Such highly interdisciplinary collaborations ensure fundamental, in depth coverage of the wide-ranging challenges posed by astrophysics. These challenges include designing astrophysically focused experiments and data analysis, tightly coupled with astrophysical searches spanning 2 orders of magnitude in wavelength, and detailed theoretical modeling. The impact of our laboratory has been particularly effective as there is constant cross-talk and feedback between quantum theorists; theoretical astrophysicists and chemists; experimental physicists; organic, physical and petroleum chemists; and infrared and UV/Vis astronomers. In this paper, two examples of the Ames Program will be given. We have been involved in identifying 9 out of the 14 interstellar pre-cometary ice species known, determined their abundances and the physical nature of the ice structure. Details on our ice work are given in the paper by Sandford et al. Our group is among the pioneers of the PAH model. We built the theoretical framework, participated in the observations and developed the experimental techniques needed to test the model. We demonstrated that the ubiquitous infrared emission spectrum associated with many interstellar objects can be matched by laboratory spectra of neutral and positively charged PAHs and that PAHs were excellent candidates for the diffuse interstellar band (DIB) carriers. See Salama et al. and Hudgins et al.

Peeling the astronomical onion.

PubMed

Rosu-Finsen, Alexander; Marchione, Demian; Salter, Tara L; Stubbing, James W; Brown, Wendy A; McCoustra, Martin R S

2016-11-23

Water ice is the most abundant solid in the Universe. Understanding the formation, structure and multiplicity of physicochemical roles for water ice in the cold, dense interstellar environments in which it is predominantly observed is a crucial quest for astrochemistry as these are regions active in star and planet formation. Intuitively, we would expect the mobility of water molecules deposited or synthesised on dust grain surfaces at temperatures below 50 K to be very limited. This work delves into the thermally-activated mobility of H 2 O molecules on model interstellar grain surfaces. The energy required to initiate this process is studied by reflection-absorption infrared spectroscopy of small quantities of water on amorphous silica and highly oriented pyrolytic graphite surfaces as the surface is annealed. Strongly non-Arrhenius behaviour is observed with an activation energy of 2 kJ mol -1 on the silica surface below 25 K and 0 kJ mol -1 on both surfaces between 25 and 100 K. The astrophysical implication of these results is that on timescales shorter than that estimated for the formation of a complete monolayer of water ice on a grain, aggregation of water ice will result in a non-uniform coating of water, hence leaving bare grain surface exposed. Other molecules can thus be formed or adsorbed on this bare surface.

Intersstellar absorption lines between 2000 and 3000 A in nearby stars observed with BUSS. [Balloon Borne Ultraviolet Spectrophotometer

NASA Technical Reports Server (NTRS)

De Boer, K. S.; Lenhart, H.; Van Der Hucht, K. A.; Kamperman, T. M.; Kondo, Y.

1986-01-01

Spectra obtained between 2000 and 3000 A with the Balloon Borne Ultraviolet Spectrophotometer (BUSS) payload were examined for interstellar absorption lines. In bright stars, with spectral types between O9V and F5V, such lines were measured of Mg I, Mg II, Cr II, Mn II, Fe II and Zn II, with Cr II and Zn II data of especially high quality. Column densities were derived and interstellar abundances were determined for the above species. It was found that metal depletion increases with increasing E(B-V); Fe was most affected and Zn showed a small depletion for E(B-V) greater than 0.3 towards Sco-Oph. The metal column densities, derived for Alpha-And, Kappa-Dra, Alpha-Com, Alpha-Aql, and 29 Cyg were used to infer N(H I). It was shown that the ratio of Mg I to Na I is instrumental in determining the ionization structure along each line of sight. The spectra of Aql stars confirms the presence of large gas densities near Alpha-Oph. Moreover, data indicated that the Rho-Oph N(H I) value needs to be altered to 35 x 10 to the 20th/sq cm, based on observed ion ratios and analysis of the Copernicus L-alpha profile.

Galactic neutral hydrogen and the magnetic ISM foreground

NASA Astrophysics Data System (ADS)

Clark, S. E.

2018-05-01

The interstellar medium is suffused with magnetic fields, which inform the shape of structures in the diffuse gas. Recent high-dynamic range observations of Galactic neutral hydrogen, combined with novel data analysis techniques, have revealed a deep link between the morphology of neutral gas and the ambient magnetic field. At the same time, an observational revolution is underway in low-frequency radio polarimetry, driven in part by the need to characterize foregrounds to the cosmological 21-cm signal. A new generation of experiments, capable of high angular and Faraday depth resolution, are revealing complex filamentary structures in diffuse polarization. The relationship between filamentary structures observed in radio-polarimetric data and those observed in atomic hydrogen is not yet well understood. Multiwavelength observations will enable new insights into the magnetic interstellar medium across phases.

Copernicus observations of C I and CO in diffuse interstellar clouds

NASA Technical Reports Server (NTRS)

Jenkins, E. B.; Jura, M.; Loewenstein, M.

1980-01-01

Copernicus was used to observe absorption lines of C I in its ground state and excited fine structure levels and CO toward 29 stars. We use the C I data to infer densities and pressures within the observed clouds, and because our results are of higher precision than previous work, much more precise estimates of the physical conditions in clouds are obtained. In agreement with previous work, the interstellar thermal pressure appears to be variable, with most clouds having values of p/k between 1000/cu cm K and 10,000/cu cm K, but there are some clouds with p/k as high as 100,000/cu cm K. Our results are consistent with the view that the interstellar thermal pressure is so variable that the gas undergoes continuous dynamic evolution. Our observations provide useful constraints on the physical processes on the surfaces of grains. In particular, we find that grains are efficient catalysts of interstellar H2 in the sense that at least half of the hydrogen atoms that strike grains come off as part of H2. Results place strong constraints on models for the formation and destruction of interstellar CO. In many clouds, an order of magnitude less CO than predicted in some models was found.

Interstellar holography

NASA Astrophysics Data System (ADS)

Walker, M. A.; Koopmans, L. V. E.; Stinebring, D. R.; van Straten, W.

2008-08-01

The dynamic spectrum of a radio pulsar is an in-line digital hologram of the ionized interstellar medium. It has previously been demonstrated that such holograms permit image reconstruction, in the sense that one can determine an approximation to the complex electric field values as a function of Doppler shift and delay, but to date the quality of the reconstructions has been poor. Here we report a substantial improvement in the method which we have achieved by simultaneous optimization of the thousands of coefficients that describe the electric field. For our test spectrum of PSRB0834+06 we find that the model provides an accurate representation of the data over the full 63dB dynamic range of the observations: residual differences between model and data are noise like. The advent of interstellar holography enables detailed quantitative investigation of the interstellar radio-wave propagation paths for a given pulsar at each epoch of observation. We illustrate this using our test data which show the scattering material to be structured and highly anisotropic. The temporal response of the medium exhibits a scattering tail which extends to beyond 100μs, and the centroid of the pulse at this frequency and this epoch of observation is delayed by approximately 15μs as a result of multipath propagation in the interstellar medium.

Exploiting single photon vacuum ultraviolet photoionization to unravel the synthesis of complex organic molecules in interstellar ices

NASA Astrophysics Data System (ADS)

Abplanalp, Matthew J.; Förstel, Marko; Kaiser, Ralf I.

2016-01-01

Complex organic molecules (COM) such as aldehydes, ketones, carboxylic acids, esters, and amides are ubiquitous in the interstellar medium, but traditional gas phase astrochemical models cannot explain their formation routes. By systematically exploiting on line and in situ vacuum ultraviolet photoionization coupled with reflectron time of flight mass spectrometry (PI-ReTOF-MS) and combining these data with infrared spectroscopy (FTIR), we reveal that complex organic molecules can be synthesized within interstellar ices that are condensed on interstellar grains via non-equilibrium reactions involving suprathermal hydrogen atoms at temperatures as low as 5 K. By probing for the first time specific structural isomers without their degradation (fragment-free), the incorporation of tunable vacuum ultraviolet photoionization allows for a much greater understanding of reaction mechanisms that exist in interstellar ices compared to traditional methods, thus eliminating the significant gap between observational and laboratory data that existed for the last decades. With the commission of the Atacama Large Millimeter/Submillimeter Array (ALMA), the number of detections of more complex organic molecules in space will continue to grow ⿿ including biorelevant molecules connected to the Origins of Life theme ⿿ and an understanding of these data will rely on future advances in sophisticated physical chemistry laboratory experiments.

Modeling the Variable Heliopause Location

NASA Astrophysics Data System (ADS)

Hensley, Kerry

2018-03-01

In 2012, Voyager 1 zipped across the heliopause. Five and a half years later, Voyager 2 still hasnt followed its twin into interstellar space. Can models of the heliopause location help determine why?How Far to the Heliopause?Artists conception of the heliosphere with the important structures and boundaries labeled. [NASA/Goddard/Walt Feimer]As our solar system travels through the galaxy, the solar outflow pushes against the surrounding interstellar medium, forming a bubble called the heliosphere. The edge of this bubble, the heliopause, is the outermost boundary of our solar system, where the solar wind and the interstellar medium meet. Since the solar outflow is highly variable, the heliopause is constantly moving with the motion driven by changes inthe Sun.NASAs twin Voyager spacecraft were poisedto cross the heliopause after completingtheir tour of the outer planets in the 1980s. In 2012, Voyager 1 registered a sharp increase in the density of interstellar particles, indicating that the spacecraft had passed out of the heliosphere and into the interstellar medium. The slower-moving Voyager 2 was set to pierce the heliopause along a different trajectory, but so far no measurements have shown that the spacecraft has bid farewell to oursolar system.In a recent study, ateam of scientists led by Haruichi Washimi (Kyushu University, Japan and CSPAR, University of Alabama-Huntsville) argues that models of the heliosphere can help explain this behavior. Because the heliopause location is controlled by factors that vary on many spatial and temporal scales, Washimiand collaborators turn to three-dimensional, time-dependent magnetohydrodynamics simulations of the heliosphere. In particular, they investigate how the position of the heliopause along the trajectories of Voyager 1 and Voyager 2 changes over time.Modeled location of the heliopause along the paths of Voyagers 1 (blue) and 2 (orange). Click for a closer look. The red star indicates the location at which Voyager 1 crossed the heliopause. The current location of Voyager 2 is marked with a red circle. [Washimi et al. 2017]A Time-Varying BarrierThe authorsconsider the impact that solar flares, coronal mass ejections, and other disturbances in the solar outflow have on the heliopause distance. These solar disturbances intermingle as they travel outward to form what the authors call global merged interaction regions.Using their hydrodynamical simulations, Washimi and collaborators capture the complex behavior of the global merged interaction regions as they propagate through the termination shock and collide with the heliopause. Part of the shock is transmitted into the local interstellar medium, while part of it is reflected back toward and collides with the termination shock, which is pushed toward the Sun. This complex interplay of transmitted and reflected shocks combined with the nonuniformity of the local interstellar medium causes the heliopause location to vary dramatically in time as well as space.What Does this Mean for Voyager 2?Washimi and collaborators find that the location of the heliopause along the trajectories of Voyagers 1 and 2 has changed considerably over the past decade. In particular, they find that the heliopause has been pushed outward over the past few years due to an increase in the solar wind ram pressure. According to their simulations, Voyager 2 is currently traveling outward faster than the heliopause is advancing, which means that the spacecraft should soon cross the boundary perhaps even thisyear to become Earths second interstellar messenger.CitationHaruichi Washimi et al 2017 ApJL 846 L9. doi:10.3847/2041-8213/aa8556

Habitable zones exposed: astrosphere collapse frequency as a function of stellar mass.

PubMed

Smith, David S; Scalo, John M

2009-09-01

Stellar astrospheres--the plasma cocoons carved out of the interstellar medium by stellar winds--are one of several buffers that partially screen planetary atmospheres and surfaces from high-energy radiation. Screening by astrospheres is continually influenced by the passage of stars through the fluctuating density field of the interstellar medium (ISM). The most extreme events occur inside dense interstellar clouds, where the increased pressure may compress an astrosphere to a size smaller than the liquid-water habitable-zone distance. Habitable planets then enjoy no astrospheric buffering from exposure to the full flux of galactic cosmic rays and interstellar dust and gas, a situation we call "descreening" or "astrospheric collapse." Under such conditions the ionization fraction in the atmosphere and contribution to radiation damage of putative coding organisms at the surface would increase significantly, and a series of papers have suggested a variety of global responses to descreening. These possibilities motivate a more careful calculation of the frequency of descreening events. Using a ram-pressure balance model, we compute the size of the astrosphere in the apex direction as a function of parent-star mass and velocity and ambient interstellar density, emphasizing the importance of gravitational focusing of the interstellar flow. The interstellar densities required to descreen planets in the habitable zone of solar- and subsolar-mass stars are found to be about 600(M/M[middle dot in circle])(-2) cm(-3) for the Sun's velocity relative to the local ISM. Such clouds are rare and small, indicating that descreening encounters are rare. We use statistics from two independent catalogues of dense interstellar clouds to derive a dependence of descreening frequency on the parent-star mass that decreases strongly with decreasing stellar mass, due to the weaker gravitational focusing and smaller habitable-zone distances for lower-mass stars. We estimate an uncertain upper limit to the absolute frequency of descreening encounters as 1-10 Gyr(-1) for solar-type stars and 10(2) to 10(9) times smaller for stars between 0.5 and 0.1 M[middle dot in circle]. Habitable-zone planets orbiting late-K to M stars are virtually never exposed to the severe consequences that have been proposed for astrospheric descreening events, but descreening events at a moderate rate may occur for stars with the Sun's mass or larger.

Thermal non-equilibrium effect of small-scale structures in compressible turbulence

NASA Astrophysics Data System (ADS)

Li, Shi-Yi; Li, Qi-Bing

2018-05-01

The thermal non-equilibrium effect of the small-scale structures in the canonical two-dimensional turbulence is studied. Comparative studies of Unified Gas Kinetic Scheme (UGKS) and GKS-Navier-Stokes (NS) for Taylor-Green flow with initial Ma = 1, Kn = 0.01 and decaying isotropic turbulence with initial Mat = 1, Reλ = 20 show that the discrepancy exists both in small and large scales, even beyond the dissipation range to 10η with accuracy to 8% in the SGS energy transfer of the decaying isotropic turbulence, illustrating the necessity for resolving the kinetic scales even at moderated Reλ = 20.

Dissipative structures in magnetorotational turbulence

NASA Astrophysics Data System (ADS)

Ross, Johnathan; Latter, Henrik N.

2018-07-01

Via the process of accretion, magnetorotational turbulence removes energy from a disc's orbital motion and transforms it into heat. Turbulent heating is far from uniform and is usually concentrated in small regions of intense dissipation, characterized by abrupt magnetic reconnection and higher temperatures. These regions are of interest because they might generate non-thermal emission, in the form of flares and energetic particles, or thermally process solids in protoplanetary discs. Moreover, the nature of the dissipation bears on the fundamental dynamics of the magnetorotational instability (MRI) itself: local simulations indicate that the large-scale properties of the turbulence (e.g. saturation levels and the stress-pressure relationship) depend on the short dissipative scales. In this paper we undertake a numerical study of how the MRI dissipates and the small-scale dissipative structures it employs to do so. We use the Godunov code RAMSES and unstratified compressible shearing boxes. Our simulations reveal that dissipation is concentrated in ribbons of strong magnetic reconnection that are significantly elongated in azimuth, up to a scale height. Dissipative structures are hence meso-scale objects, and potentially provide a route by which large scales and small scales interact. We go on to show how these ribbons evolve over time - forming, merging, breaking apart, and disappearing. Finally, we reveal important couplings between the large-scale density waves generated by the MRI and the small-scale structures, which may illuminate the stress-pressure relationship in MRI turbulence.

Observational evidence of dust evolution in galactic extinction curves

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cecchi-Pestellini, Cesare; Casu, Silvia; Mulas, Giacomo

Although structural and optical properties of hydrogenated amorphous carbons are known to respond to varying physical conditions, most conventional extinction models are basically curve fits with modest predictive power. We compare an evolutionary model of the physical properties of carbonaceous grain mantles with their determination by homogeneously fitting observationally derived Galactic extinction curves with the same physically well-defined dust model. We find that a large sample of observed Galactic extinction curves are compatible with the evolutionary scenario underlying such a model, requiring physical conditions fully consistent with standard density, temperature, radiation field intensity, and average age of diffuse interstellar clouds.more » Hence, through the study of interstellar extinction we may, in principle, understand the evolutionary history of the diffuse interstellar clouds.« less

Interstellar fullerene compounds and diffuse interstellar bands

NASA Astrophysics Data System (ADS)

Omont, Alain

2016-05-01

Recently, the presence of fullerenes in the interstellar medium (ISM) has been confirmed and new findings suggest that these fullerenes may possibly form from polycyclic aromatic hydrocarbons (PAHs) in the ISM. Moreover, the first confirmed identification of two strong diffuse interstellar bands (DIBs) with the fullerene, C60+, connects the long standing suggestion that various fullerenes could be DIB carriers. These new discoveries justify reassessing the overall importance of interstellar fullerene compounds, including fullerenes of various sizes with endohedral or exohedral inclusions and heterofullerenes (EEHFs). The phenomenology of fullerene compounds is complex. In addition to fullerene formation in grain shattering, fullerene formation from fully dehydrogenated PAHs in diffuse interstellar clouds could perhaps transform a significant percentage of the tail of low-mass PAH distribution into fullerenes including EEHFs. But many uncertain processes make it extremely difficult to assess their expected abundance, composition and size distribution, except for the substantial abundance measured for C60+. EEHFs share many properties with pure fullerenes, such as C60, as regards stability, formation/destruction and chemical processes, as well as many basic spectral features. Because DIBs are ubiquitous in all lines of sight in the ISM, we address several questions about the interstellar importance of various EEHFs, especially as possible carriers of diffuse interstellar bands. Specifically, we discuss basic interstellar properties and the likely contributions of fullerenes of various sizes and their charged counterparts such as C60+, and then in turn: 1) metallofullerenes; 2) heterofullerenes; 3) fulleranes; 4) fullerene-PAH compounds; 5) H2@C60. From this reassessment of the literature and from combining it with known DIB line identifications, we conclude that the general landscape of interstellar fullerene compounds is probably much richer than heretofore realized. EEHFs, together with pure fullerenes of various sizes, have many properties necessary to be suitably carriers of DIBs: carbonaceous nature; stability and resilience in the harsh conditions of the ISM; existing with various heteroatoms and ionization states; relatively easy formation; few stable isomers; spectral lines in the right spectral range; various and complex energy internal conversion; rich Jahn-Teller fine structure. This is supported by the first identification of a DIB carrier as C60+. Unfortunately, the lack of any precise information about the complex optical spectra of EEHFs and most pure fullerenes other than C60 and about their interstellar abundances still precludes definitive assessment of the importance of fullerene compounds as DIB carriers. Their compounds could significantly contribute to DIBs, but it still seems difficult that they are the only important DIB carriers. Regardless, DIBs appear as the most promising way of tracing the interstellar abundances of various fullerene compounds if the breakthrough in identifying C60+ as a DIB carrier can be extended to more spectral features through systematic studies of their laboratory gas-phase spectroscopy.

Observations of Carbon Isotopic Fractionation in Interstellar Formaldehyde

NASA Technical Reports Server (NTRS)

Wirstrom, E. S.; Charnley, S. B.; Geppert, W. D.; Persson, C. M.

2012-01-01

Primitive Solar System materials (e.g. chondrites. IDPs, the Stardust sample) show large variations in isotopic composition of the major volatiles (H, C, N, and O ) even within samples, witnessing to various degrees of processing in the protosolar nebula. For ex ample. the very pronounced D enhancements observed in IDPs [I] . are only generated in the cold. dense component of the interstellar medium (ISM), or protoplanetary disks, through ion-molecule reactions in the presence of interstellar dust. If this isotopic anomaly has an interstellar origin, this leaves open the possibility for preservation of other isotopic signatures throughout the form ation of the Solar System. The most common form of carbon in the ISM is CO molecules, and there are two potential sources of C-13 fractionation in this reservoir: low temperature chemistry and selective photodissociation. While gas-phase chemistry in cold interstellar clouds preferentially incorporates C-13 into CO [2], the effect of self-shielding in the presence of UV radiation instead leads to a relative enhancement of the more abundant isotopologue, 12CO. Solar System organic material exhibit rather small fluctuations in delta C-13 as compared to delta N-15 and delta D [3][1], the reason for which is still unclear. However, the fact that both C-13 depleted and enhanced material exists could indicate an interstellar origin where the two fractionation processes have both played a part. Formaldehyde (H2CO) is observed in the gas-phase in a wide range of interstellar environments, as well as in cometary comae. It is proposed as an important reactant in the formation of more complex organic molecules in the heated environments around young stars, and formaldehyde polymers have been suggested as the common origin of chondritic insoluable organic matter (IOM) and cometary refractory organic solids [4]. The relatively high gas-phase abundance of H2CO observed in molecular clouds (10(exp- 9) - 10(exp- 8) relative to H2) makes it feasible to observe its less common isotopologues. As a step in our investigation of C-13 fractionation patterns in the ISM, we here present comparisons between observations of the C-13 fraction in formaldehyde, and chemical fractionation models.

Gas clump formation via thermal instability in high-redshift dwarf galaxy mergers

NASA Astrophysics Data System (ADS)

Arata, Shohei; Yajima, Hidenobu; Nagamine, Kentaro

2018-04-01

Star formation in high-redshift dwarf galaxies is a key to understand early galaxy evolution in the early Universe. Using the three-dimensional hydrodynamics code GIZMO, we study the formation mechanism of cold, high-density gas clouds in interacting dwarf galaxies with halo masses of ˜3 × 107 M⊙, which are likely to be the formation sites of early star clusters. Our simulations can resolve both the structure of interstellar medium on small scales of ≲ 0.1 pc and the galactic disc simultaneously. We find that the cold gas clouds form in the post-shock region via thermal instability due to metal-line cooling, when the cooling time is shorter than the galactic dynamical time. The mass function of cold clouds shows almost a power-law initially with an upper limit of thermally unstable scale. We find that some clouds merge into more massive ones with ≳104 M⊙ within ˜ 2 Myr. Only the massive cold clouds with ≳ 103 M⊙ can keep collapsing due to gravitational instability, resulting in the formation of star clusters. We find that the clump formation is more efficient in the prograde-prograde merger than the prograde-retrograde case due to the difference in the degree of shear flow. In addition, we investigate the dependence of cloud mass function on metallicity and H2 abundance, and show that the cases with low metallicities (≲10-2 Z⊙) or high H2 abundance (≳10-3) cannot form massive cold clouds with ≳103 M⊙.

The Potential Role Played by the Fullerene-Like Structures of Interstellar Carbon Dust in the Formation of Molecular Hydrogen in Space

NASA Astrophysics Data System (ADS)

Cataldo, Franco; Iglesias-Groth, Susana

After a general introduction to the problem of formation of molecular hydrogen from atomic hydrogen in the interstellar medium and in the dense molecular clouds in particular, and after the explanation of the key role played by the surfaces on this process, it is proposed that the most suitable carbon surface for the formation of molecular hydrogen (from the radiative association process of atomic hydrogen) can be represented by carbon black rather than by graphite. Furthermore, it is proposed that the fullerene-like structures present in the carbon black graphene sheets are the reaction sites where molecular hydrogen may be formed.

Observational properties of pulsars.

PubMed

Manchester, R N

2004-04-23

Pulsars are remarkable clocklike celestial sources that are believed to be rotating neutron stars formed in supernova explosions. They are valuable tools for investigations into topics such as neutron star interiors, globular cluster dynamics, the structure of the interstellar medium, and gravitational physics. Searches at radio and x-ray wavelengths over the past 5 years have resulted in a large increase in the number of known pulsars and the discovery of new populations of pulsars, posing challenges to theories of binary and stellar evolution. Recent images at radio, optical, and x-ray wavelengths have revealed structures resulting from the interaction of pulsar winds with the surrounding interstellar medium, giving new insights into the physics of pulsars.

Diffuse cloud chemistry. [in interstellar matter

NASA Technical Reports Server (NTRS)

Van Dishoeck, Ewine F.; Black, John H.

1988-01-01

The current status of models of diffuse interstellar clouds is reviewed. A detailed comparison of recent gas-phase steady-state models shows that both the physical conditions and the molecular abundances in diffuse clouds are still not fully understood. Alternative mechanisms are discussed and observational tests which may discriminate between the various models are suggested. Recent developments regarding the velocity structure of diffuse clouds are mentioned. Similarities and differences between the chemistries in diffuse clouds and those in translucent and high latitude clouds are pointed out.

Prospects for Studying Interstellar Magnetic Fields with a Far-Infrared Polarimeter for SAFIR

NASA Technical Reports Server (NTRS)

Dowell, C. Darren; Chuss, D. T.; Dotson, J. L.

2008-01-01

Polarimetry at mid-infrared through millimeter wavelengths using airborne and ground-based telescopes has revealed magnetic structures in dense molecular clouds in the interstellar medium, primarily in regions of star formation. Furthermore, spectropolarimetry has offered clues about the composition of the dust grains and the mechanism by which they are aligned with respect to the local magnetic field. The sensitivity of the observations to date has been limited by the emission from the atmosphere and warm telescopes. A factor of 1000 in sensitivity can be gained by using instead a cold space telescope. With 5 arcminute resolution, Planck will make the first submillimeter polarization survey of the full Galaxy early in the next decade. We discuss the science case for and basic design of a far-infrared polarimeter on the SAFIR space telescope, which offers resolution in the few arcsecond range and wavelength selection of cold and warm dust components. Key science themes include the formation and evolution of molecular clouds in nearby spiral galaxies, the magnetic structure of the Galactic center, and interstellar turbulence.

Properties and Alignment of Interstellar Dust Grains toward Type Ia Supernovae with Anomalous Polarization Curves

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hoang, Thiem, E-mail: thiemhoang@kasi.re.kr; Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8; Institute of Theoretical Physics, Goethe Universität Frankfurt, D-60438 Frankfurt am Main

Recent photometric and polarimetric observations of Type Ia supernovae (SNe Ia) show unusually low total-to-selective extinction ratios ( R {sub V} < 2) and wavelengths of maximum polarization ( λ{sub max} < 0.4 μ m) for several SNe Ia, which indicates peculiar properties of interstellar (IS) dust in the SN-hosted galaxies and/or the presence of circumstellar (CS) dust. In this paper, we use an inversion technique to infer the best-fit grain size distribution and the alignment function of interstellar grains along the lines of sight toward four SNe Ia with anomalous extinction and polarization data (SN 1986G, SN 2006X, SNmore » 2008fp, and SN 2014J). We find that to reproduce low values of R{sub V}, a significant enhancement in the mass of small grains of radius a < 0.1 μ m is required. For SN 2014J, a simultaneous fit to its observed extinction and polarization is unsuccessful if all the data are attributed to IS dust (model 1), but a good fit is obtained when accounting for the contribution of CS dust (model 2). For SN 2008fp, our best-fit results for model 1 show that in order to reproduce an extreme value of λ{sub max} ∼ 0.15 μ m, small silicate grains must be aligned as efficiently as big grains. For this case, we suggest that strong radiation from the SN can induce efficient alignment of small grains in a nearby intervening molecular cloud via the radiative torque (RAT) mechanism. The resulting time dependence polarization from this RAT alignment model can be tested by observing at ultraviolet wavelengths.« less

Equatorial Density Irregularity Structures at Intermediate Scales and Their Temporal Evolution

NASA Technical Reports Server (NTRS)

Kil, Hyosub; Heelis, R. A.

1998-01-01

We examine high resolution measurements of ion density in the equatorial ionosphere from the AE-E satellite during the years 1977-1981. Structure over spatial scales from 18 km to 200 m is characterized by the spectrum of irregularities at larger and smaller scales and at altitudes above 350 km and below 300 km. In the low-altitude region, only small amplitude large-scale (lambda greater than 5 km) density modulations are often observed, and thus the power spectrum of these density structures exhibits a steep spectral slope at kilometer scales. In the high-altitude region, sinusoidal density fluctuations, characterized by enhanced power near 1-km scale, are frequently observed during 2000-0200 LT. However, such fluctuations are confined to regions at the edges of larger bubble structures where the average background density is high. Small amplitude irregularity structures, observed at early local time hours, grow rapidly to high-intensity structures in about 90 min. Fully developed structures, which are observed at late local time hours, decay very slowly producing only-small differences in spectral characteristics even 4 hours later. The local time evolution of irregularity structure is investigated by using average statistics for low-(1% less than sigma less than 5%) and high-intensity (sigma greater than 10%) structures. At lower altitudes, little chance in the spectral slope is seen as a function of local time, while at higher attitudes the growth and maintenance of structures near 1 km scales dramatically affects the spectral slope.

Million-degree plasma pervading the extended Orion Nebula.

PubMed

Güdel, Manuel; Briggs, Kevin R; Montmerle, Thierry; Audard, Marc; Rebull, Luisa; Skinner, Stephen L

2008-01-18

Most stars form as members of large associations within dense, very cold (10 to 100 kelvin) molecular clouds. The nearby giant molecular cloud in Orion hosts several thousand stars of ages less than a few million years, many of which are located in or around the famous Orion Nebula, a prominent gas structure illuminated and ionized by a small group of massive stars (the Trapezium). We present x-ray observations obtained with the X-ray Multi-Mirror satellite XMM-Newton, revealing that a hot plasma with a temperature of 1.7 to 2.1 million kelvin pervades the southwest extension of the nebula. The plasma flows into the adjacent interstellar medium. This x-ray outflow phenomenon must be widespread throughout our Galaxy.

Hot interstellar tunnels. 1: Simulation of interacting supernova remnants

NASA Technical Reports Server (NTRS)

Smith, B. W.

1976-01-01

The theory required to build a numerical simulation of interacting supernova remnants is developed. The hot cavities within a population of remnants will become connected, with varying ease and speed, for a variety of assumed conditions in the outer shells of old remnants. Apparently neither radiative cooling nor thermal conduction in a large-scale galactic magnetic field can destroy hot cavity regions, if they grow, faster than they are reheated by supernova shock waves, but interstellar mass motions disrupt the contiguity of extensive cavities necessary for the dispersal of these shocks over a wide volume. Monte Carlo simulations show that a quasi-equilibrium is reached in the test space within 10 million yrs of the first supernova and is characterized by an average cavity filling fraction of the interstellar volume. Aspects of this equilibrium are discussed for a range of supernova rates. Two predictions are not confirmed within this range: critical growth of hot regions to encompass the entire medium, and the efficient quenching of a remnant's expansion by interaction with other cavities.

The Emergence of the Worldship (I): The Shift from Planet-Based to Space-Based Civilisation

NASA Astrophysics Data System (ADS)

Ashworth, S.

Design concepts for passenger-carrying interstellar vehicles may be organised according to speed of travel and payload mass. The most likely design solutions fall on a scale which ranges from the high speed, low mass rapid transport at one end to the low speed, high mass multi-generation worldship at the other. The medium speed, medium mass cruiser is defined as an intermediate case. Using an energy-based analysis, it is shown that the rapid transport is a less plausible case. The more credible options for human interstellar flight are the multi-generation cruiser and worldship, in either case requiring the construction of an artificial mobile world-like environment for the sustainable support of a town- to city-sized community of travellers. This could be made possible by a shift in the dominant mode of human civilisation from planetary to space-based life. The long-term consequences for interstellar colonisation are illustrated with reference to the percolation theory presented by Geoffrey Landis.

Origins Space Telescope: Nearby Galaxies, the Milky Way, and the Interstellar Medium

NASA Astrophysics Data System (ADS)

Battersby, Cara; Sandstrom, Karin; Origins Space Telescope Science and Technology Definition Team

2018-01-01

The Origins Space Telescope (OST) is the mission concept for the Far-Infrared Surveyor, one of the four science and technology definition studies of NASA Headquarters for the 2020 Astronomy and Astrophysics Decadal survey. Origins will enable flagship-quality general observing programs led by the astronomical community in the 2030s. We welcome you to contact the Science and Technology Definition Team (STDT) with your science needs and ideas by emailing us at ost_info@lists.ipac.caltech.eduThis presentation will summarize the science case related to Nearby Galaxies, the Milky Way, and the Interstellar Medium (Interstellar Medium). The Origins Space Telescope will enable a wealth of unprecedented scientific advances in this area, both those we know to expect, and the discovery space that lies unexplored. Origins will enable a comprehensive view of magnetic fields, turbulence, and the multiphase ISM; connecting these physics across scales of galaxies to protostellar cores. With unprecedented sensitivity, Origins will measure and characterize the mechanisms of feedback from star formation and Active Galactic Nuclei, and their interplay, over cosmic time. Origins will unveil the abundance and availability of water for habitable planets by allowing us to trace the trail of water from interstellar clouds to protoplanetary disks, to Earth itself.

The fate of NGC602, an intense region of star-formation in the Wing of the SMC

NASA Astrophysics Data System (ADS)

Sabbi, Elena

2017-08-01

This is a small 2 orbit proposal designed to measure the internal dynamics of NGC602, a small region of intense star formation in the Wing of the SMC, with a low gas and dust density that has been often considered an unfavorable place for star formation. Small regions of massive star formation are important to study for our understanding of the process of star and cluster formation, the ionization of the interstellar medium, and the injection of energy and momentum into their host galaxy. By combining our new observations with archival ACS/WFC data acquired in July 2004, we will be able to measure the relative proper motions of the NGC602 sub-structures better than 2.3 km/s and investigate the nature of the apparently isolated massive stars found around NGC602. This study will provide unique observational data to characterize the early phase of cluster evolution and test cluster formation theories. It will also address significant open issues in star formation, cluster dynamics and the origin of isolated supernovae and GRBs.

The Structure of the Heliosphere as Seen from In Situ and Remote Observations

NASA Astrophysics Data System (ADS)

Pogorelov, N. V.; Heerikhuisen, J.; Kim, T. K.; Zhang, M.

2017-12-01

The heliosphere is formed due to interaction between the solar wind (SW) and local interstellar medium (LISM). The shape and position of the heliospheric boundary, the heliopause, in space depend on the parameters of interacting plasma flows. The interplay between the asymmetrizing effect of the interstellar magnetic field and charge exchange between ions and neutral atoms plays an important role in the SW-LISM interaction. By performing three-dimensional, MHD plasma / kinetic neutral atom simulations, we describe the structure of the outer heliosheath (OHS) - the LISM plasma region affected by the presence of the heliosphere - and analyze quantitatively the distributions in front of the heliopause. It is shown that charge exchange modifies the LISM plasma to such extent that the contribution of a shock transition to the total variation of plasma parameters becomes small even if the LISM velocity exceeds the fast magnetosonic speed in the unperturbed medium. By performing adaptive mesh refinement simulations, we show that a distinct boundary layer of decreased plasma density and enhanced magnetic field should be observed on the interstellar side of the heliopause. We show that this behavior is in agreement with the plasma oscillations of increasing frequency observed by the plasma wave instrument onboard Voyager 1. Numerical results are presented that reproduce shocks that pass by Voyager 1 in the OHS. We demonstrate that Voyager observations in the inner heliosheath between the heliospheric termination shock and the heliopause are consistent with dissipation of the heliospheric magnetic field. The effect of pickup ions is discussed in the context of in situ measurements. We also show that multi-TeV cosmic ray anisotropy can serve as an imager of the heliosphere due to its effect on the LISM properties. In particular, both the bow wave and the heliotail reveal themselves as characteristic features in the TeV flux images. The choice of LISM parameters in this analysis is based on the simulations that fit observations of energetic neutral atoms performed by IBEX. In particular, the position of the IBEX ribbon on the celestial sphere is strongly dependent on the choice of the plane that contains the vectors of velocity and magnetic field in the unperturbed LISM.

Exotic Molecules in Space: A Coordinated Astronomical Laboratory and Theoretical Study

NASA Technical Reports Server (NTRS)

Thaddeus, Patrick

1999-01-01

The past three years have been a period of great progress in our laboratory investigation of molecules of astrophysical interest-the most productive by far in the 20-year history of a research program which has led to the discovery of over 20% of the 123 known interstellar and circumstellar molecules. Most of the discoveries made during this period have been the result of the construction in late 1995 and early 1996 of a Fourier transform microwave spectrometer working in the centimeter-wave band. The sensitivity of this instrument from the moment that it was turned on has exceeded our expectations by an order of magnitude. The Table below shows the 46 new molecules which have been discovered. Most are carbon chains, the dominant type of molecule which has been found in space. Several comments with respect to these molecules should be made: 1. There are probably no mistakes in any of the identifications, since these have been confirmed by the standard, powerful assays and tests used to check spectroscopic identifications: isotopic substitution, quantum calculations of the expected molecular structures, detection of hyperfine structure, Zeeman effect, etc. 2. The radio laboratory astrophysics of the entire set is complete for the time being, in the sense that essentially all the astronomically interesting radio transitions (including hfs when present) are either directly measured or can now be calculated from the derived spectroscopic constants to better than 1 part per million (or 0.3 km s-1 in radial velocity, and often much better than that). 3. Six of the forty six new molecules have already been identified in space, in every case but one on the basis of our laboratory measurements. 4. Sensitive as they are, our laboratory techniques are far from fundamental limits on sensitivity, and 5. One of the principal motivations of our research is to close the fairly small mass and size gap, now only a factor of a few, between the smallest postulated interstellar grains and the largest identified interstellar molecules.

Matrix-Isolation Spectroscopy of Reactive Organic Molecules of Relevance to Interstellar Space

NASA Astrophysics Data System (ADS)

Kopff, Laura A.; Nolan, Alex M.; Kreifels, Terese A.; Draxler, Thomas W.; Esselman, Brian J.; Burrmann, Nicola J.; McMahon, Robert J.

2010-11-01

Matrix isolation, the process of trapping a molecule in an inert gas at low temperature, provides a means for studying highly reactive intermediates, such as carbenes or radicals. Reactive species can be characterized by IR, UV-vis and/or EPR spectroscopy. Comparison of experimental and computed spectral data, as well as chemical reactivity, is used for structural assignment Triplet propynylidene is proposed to exist in the interstellar medium (ISM), due to the detection of a higher-energy isomers via rotational spectroscopy. Currently, we are exploring the structural and photochemical effects of varying substituents on the propynylidne system. A diazo precursor has been synthesized and photolyzed to produce dimethylpropynylidene in an argon matrix. A photochemical hydrogen shift to produce 1-penten-3-yne has been observed through infrared spectroscopy. Cyanocarbons are known to be abundant in the ISM and the atmosphere of Titan, however matrixisolation studies have not yet been carried out for a significant number of these compounds. Photolysis of 3-cyano-3-methyldiazirine should yield methylcyanocarbene, one of the simplest species in this family. Another molecule of interest is l-HC4N, which has been detected in the ISM, but has not yet been matrix-isolated and characterized. The study of arylcarbenes is vital to understanding the chemistry of carbon-rich environments, such as discharges, interstellar clouds, and circumstellar envelopes. The identification of small, sulfur containing molecules, and the identification of aromatics in the ISM make future thiophene and benzothiophene detections a real possibility. Studies on 2- and 3-diazomethyl substituted benzothiophenes are underway to assess their photochemical reactivity and potential for forming benzothiophene carbenes. Macrocylic polyynes are proposed to be involved in carbon condensation via the ring coalescence and annealing model to produce graphitic sheets or fullerenes. To simplify a complex system we are experimentally and computationally studying the series of ethynyl-substituted cyclobutadienes and their possible involvement in the build-up of larger carbon containing molecules in the ISM. The Bergman cyclization of cyclobutadiene has been explored computationally and the photochemical precursor is currently being synthesized.

Planck intermediate results: XXXVIII. E- and B-modes of dust polarization from the magnetized filamentary structure of the interstellar medium

DOE PAGES

Ade, P. A. R.; Aghanim, N.; Arnaud, M.; ...

2016-02-09

The quest for a B-mode imprint from primordial gravity waves on the polarization of the cosmic microwave background (CMB) requires the characterization of foreground polarization from Galactic dust. In this paper, we present a statistical study of the filamentary structure of the 353 GHz Planck Stokes maps at high Galactic latitude, relevant to the study of dust emission as a polarized foreground to the CMB. We filter the intensity and polarization maps to isolate filaments in the range of angular scales where the power asymmetry between E-modes and B-modes is observed. Using the Smoothed Hessian Major Axis Filament Finder (SMAFF),more » we identify 259 filaments at high Galactic latitude, with lengths larger or equal to 2° (corresponding to 3.5 pc in length for a typical distance of 100 pc). Thesefilaments show a preferred orientation parallel to the magnetic field projected onto the plane of the sky, derived from their polarization angles. We present mean maps of the filaments in Stokes I, Q, U, E, and B, computed by stacking individual images rotated to align the orientations of the filaments. Combining the stacked images and the histogram of relative orientations, we estimate the mean polarization fraction of the filaments to be 11%. Furthermore, we show that the correlation between the filaments and the magnetic field orientations may account for the E and B asymmetry and the C ℓ TE/C ℓ EE ratio, reported in the power spectra analysis of the Planck353 GHz polarization maps. Finally, future models of the dust foreground for CMB polarization studies will need to take into account the observed correlation between the dust polarization and the structure of interstellar matter.« less

High Resolution FIR and IR Spectroscopy of Methanol Isotopologues

NASA Astrophysics Data System (ADS)

Lees, R. M.; Xu, Li-Hong; Appadoo, D. R. T.; Billinghurst, B.

2010-02-01

New astronomical facilities such as HIFI on the Herschel Space Observatory, the SOFIA airborne IR telescope and the ALMA sub-mm telescope array will yield spectra from interstellar and protostellar sources with vastly increased sensitivity and frequency coverage. This creates the need for major enhancements to laboratory databases for the more prominent interstellar "weed" species in order to model and account for their lines in observed spectra in the search for new and more exotic interstellar molecular "flowers." With its large-amplitude internal torsional motion, methanol has particularly rich spectra throughout the FIR and IR regions and, being very widely distributed throughout the galaxy, is perhaps the most notorious interstellar weed. Thus, we have recorded new spectra for a variety of methanol isotopic species on the high-resolution FTIR spectrometer on the CLS FIR beamline. The aim is to extend quantum number coverage of the data, improve our understanding of the energy level structure, and provide the astronomical community with better databases and models of the spectral patterns with greater predictive power for a range of astrophysical conditions.

Creation of current filaments in the solar corona

NASA Technical Reports Server (NTRS)

Mikic, Z.; Schnack, D. D.; Van Hoven, G.

1989-01-01

It has been suggested that the solar corona is heated by the dissipation of electric currents. The low value of the resistivity requires the magnetic field to have structure at very small length scales if this mechanism is to work. In this paper it is demonstrated that the coronal magnetic field acquires small-scale structure through the braiding produced by smooth, randomly phased, photospheric flows. The current density develops a filamentary structure and grows exponentially in time. Nonlinear processes in the ideal magnetohydrodynamic equations produce a cascade effect, in which the structure introduced by the flow at large length scales is transferred to smaller scales. If this process continues down to the resistive dissipation length scale, it would provide an effective mechanism for coronal heating.

Use of Mini-Mag Orion and superconducting coils for near-term interstellar transportation

NASA Astrophysics Data System (ADS)

Lenard, Roger X.; Andrews, Dana G.

2007-06-01

Interstellar transportation to nearby star systems over periods shorter than the human lifetime requires speeds in the range of 0.1-0.15 c and relatively high accelerations. These speeds are not attainable using rockets, even with advanced fusion engines because at these velocities, the energy density of the spacecraft approaches the energy density of the fuel. Anti-matter engines are theoretically possible but current physical limitations would have to be suspended to get the mass densities required. Interstellar ramjets have not proven practicable, so this leaves beamed momentum propulsion or a continuously fueled Mag-Orion system as the remaining candidates. However, deceleration is also a major issue, but part of the Mini-Mag Orion approach assists in solving this problem. This paper reviews the state of the art from a Phases I and II SBIT between Sandia National Laboratories and Andrews Space, applying our results to near-term interstellar travel. A 1000 T crewed spacecraft and propulsion system dry mass at .1c contains ˜9×1021J. The author has generated technology requirements elsewhere for use of fission power reactors and conventional Brayton cycle machinery to propel a spacecraft using electric propulsion. Here we replace the electric power conversion, radiators, power generators and electric thrusters with a Mini-Mag Orion fission-fusion hybrid. Only a small fraction of fission fuel is actually carried with the spacecraft, the remainder of the propellant (macro-particles of fissionable material with a D-T core) is beamed to the spacecraft, and the total beam energy requirement for an interstellar probe mission is roughly 1020J, which would require the complete fissioning of 1000 ton of Uranium assuming 35% power plant efficiency. This is roughly equivalent to a recurring cost per flight of 3.0 billion dollars in reactor grade enriched uranium using today's prices. Therefore, interstellar flight is an expensive proposition, but not unaffordable, if the nonrecurring costs of building the power plant can be minimized.

IBEX Mission update: New discoveries and a new orbit

NASA Astrophysics Data System (ADS)

McComas, D. J.; IBEX Science Team

2011-12-01

The Interstellar Boundary Explorer (IBEX) mission has been remotely observing the global interaction of our heliosphere with the local interstellar medium for over two and a half years. Initially, IBEX generated the first all-sky maps of Energetic Neutral Atoms (ENAs) emanating in from the boundaries of our heliosphere over the energy range from ~0.1-6 keV. Using these observations, the IBEX team discovered a smoothly varying, globally distributed ENA flux overlaid by a narrow "ribbon" of significantly enhanced ENA emissions. Since the initial publications of these results in a special issue of Science magazine (November 2009), IBEX has completed four more energy-resolved sets of sky maps and discovered small but important time variations in the interaction, separated the ribbon from globally distributed ENA fluxes, measured the energy spectral shape and inferred ion source temperatures, and carried out many other observational and theoretical studies. In addition, IBEX made the first observations of ENAs produced by backscatter and neutralization of the solar wind from the lunar regolith and provided the first energy and angle resolved ENA images of the subsolar magnetosheath and magnetospheric cusps and plasma sheet. Most recently, direct IBEX observations of Interstellar Neutral (ISN) He show that the speed and direction (the motion of the heliosphere with respect to the interstellar medium) is different than that thought from prior Ulysses observations. These observations also show evidence for a previously unknown and unanticipated secondary population of Helium. In addition, IBEX is providing the first direct quantitative measurements of the ISN H parameters and the first direct measurements of interstellar Ne and the interstellar Neon/Oxygen abundance ratio; this ratio is significantly different than the solar abundance ratio. IBEX was recently maneuvered into a unique, long-term stable orbit and has several decades worth of fuel for routine operations. Thus, IBEX will likely continue to provide revolutionary observations of numerous heliospheric, magnetospheric, and planetary phenomena for many years to come!

Dispersion of gravitational waves in cold spherical interstellar medium

NASA Astrophysics Data System (ADS)

Barta, Dániel; Vasúth, Mátyás

We investigate the propagation of locally plane, small-amplitude, monochromatic gravitational waves (GWs) through cold compressible interstellar gas in order to provide a more accurate picture of expected waveforms for direct detection. The quasi-isothermal gas is concentrated in a spherical symmetric cloud held together by self-gravitation. Gravitational waves can be treated as linearized perturbations on the background inner Schwarzschild spacetime. The perturbed quantities lead to the field equations governing the gas dynamics and describe the interaction of gravitational waves with matter. We have shown that the transport equation of these amplitudes provides numerical solutions for the frequency-alteration. The decrease in frequency is driven by the energy dissipating process of GW-matter interactions. The decrease is significantly smaller than the magnitude of the original frequency and too small to be detectable by present second-generation and planned third-generation detectors. It exhibits a power-law relationship between original and decreased frequencies. The frequency deviation was examined particularly for the transient signal GW150914.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Asahina, Yuta; Kawashima, Tomohisa; Furukawa, Naoko

The formation mechanism of CO clouds observed with the NANTEN2 and Mopra telescopes toward the stellar cluster Westerlund 2 is studied by 3D magnetohydrodynamic simulations, taking into account the interstellar cooling. These molecular clouds show a peculiar shape composed of an arc-shaped cloud on one side of the TeV γ -ray source HESS J1023-575 and a linear distribution of clouds (jet clouds) on the other side. We propose that these clouds are formed by the interaction of a jet with clumps of interstellar neutral hydrogen (H i). By studying the dependence of the shape of dense cold clouds formed bymore » shock compression and cooling on the filling factor of H i clumps, we found that the density distribution of H i clumps determines the shape of molecular clouds formed by the jet–cloud interaction: arc clouds are formed when the filling factor is large. On the other hand, when the filling factor is small, molecular clouds align with the jet. The jet propagates faster in models with small filling factors.« less

Phase transition between atomic and molecular hydrogen in nearby spiral galaxies

NASA Astrophysics Data System (ADS)

Tanaka, Ayako; Nakanishi, Hiroyuki; Kuno, Nario; Hirota, Akihiko

2014-06-01

We compared theoretical and observational molecular mass fractions (fmol: ratio of molecular gas density to total gas density) using observational data of ten nearby spiral galaxies. For determination of fmol, the three parameters-interstellar pressure P, UV radiation U, and metallicity Z-were obtained from the spectral line data of 12CO(J = 1-0), H I, Hα, [O III], and [O II]. Interstellar pressure was calculated with the sum of the hydrogen gas densities and the stellar potential based on the Ks-band data. For most data other than metallicity, we used archived NRO CO Atlas, THINGS, SINGS, and 2MASS data. For comparison, we also investigated the dependence of the CO-to-H2 conversion factor XCO. It was found that the theoretical fmol agreed with the observational fmol only when the interstellar pressure is calculated with both the gas density and stellar disk potential. To fit observations more accurately, either the metallicity or the UV radiation needs to be adjusted. It was also found that, in UV radiation scaling, scaling factor γ has a correlation with the diffuse fraction of the Hα emission line data, fDIG. As for XCO, it was shown that the difference between both values of fmol becomes the least when XCO is 1.0 × 1020 cm-2 (K km s-1)-1.

Metallicity fluctuation statistics in the interstellar medium and young stars - I. Variance and correlation

NASA Astrophysics Data System (ADS)

Krumholz, Mark R.; Ting, Yuan-Sen

2018-04-01

The distributions of a galaxy's gas and stars in chemical space encode a tremendous amount of information about that galaxy's physical properties and assembly history. However, present methods for extracting information from chemical distributions are based either on coarse averages measured over galactic scales (e.g. metallicity gradients) or on searching for clusters in chemical space that can be identified with individual star clusters or gas clouds on ˜1 pc scales. These approaches discard most of the information, because in galaxies gas and young stars are observed to be distributed fractally, with correlations on all scales, and the same is likely to be true of metals. In this paper we introduce a first theoretical model, based on stochastically forced diffusion, capable of predicting the multiscale statistics of metal fields. We derive the variance, correlation function, and power spectrum of the metal distribution from first principles, and determine how these quantities depend on elements' astrophysical origin sites and on the large-scale properties of galaxies. Among other results, we explain for the first time why the typical abundance scatter observed in the interstellar media of nearby galaxies is ≈0.1 dex, and we predict that this scatter will be correlated on spatial scales of ˜0.5-1 kpc, and over time-scales of ˜100-300 Myr. We discuss the implications of our results for future chemical tagging studies.

Recursivity in Lingua Cosmica

NASA Astrophysics Data System (ADS)

Ollongren, Alexander

2011-02-01

In a sequence of papers on the topic of message construction for interstellar communication by means of a cosmic language, the present author has discussed various significant requirements such a lingua should satisfy. The author's Lingua Cosmica is a (meta) system for annotating contents of possibly large-scale messages for ETI. LINCOS, based on formal constructive logic, was primarily designed for dealing with logic contents of messages but is also applicable for denoting structural properties of more general abstractions embedded in such messages. The present paper explains ways and means for achieving this for a special case: recursive entities. As usual two stages are involved: first the domain of discourse is enriched with suitable representations of the entities concerned, after which properties over them can be dealt with within the system itself. As a representative example the case of Russian dolls (Matrjoshka's) is discussed in some detail and relations with linguistic structures in natural languages are briefly exploited.

[Petrological Analysis of Astrophysical Dust Analog Evolution

NASA Technical Reports Server (NTRS)

Rietmeijer, Frans J. M.

1997-01-01

This project "Petrological analysis of astrophysical dust analog evolution" was initiated to try to understand the vapor phase condensation, and the nature of the reaction products, in circumstellar environments, such as the solar nebula 4,500 Myrs ago, and in the interstellar medium. Telescope-based infrared [IR] spectroscopy offers a broad-scale inventory of the various types of dust in these environments but no details on small-scale variations in terms of chemistry and morphology and petrological phase relationships. Vapor phase condensation in these environments is almost certainly a non-equilibrium process. The main challenge to this research was to document the nature of this process that, based on astrophysical observations, seems to yield compositionally consistent materials. This observation may suggest a predictable character during non-equilibrium condensation. These astrophysical environments include two chemically distinct, that is, oxygen-rich and carbon-rich environments. The former is characterized by silicates the latter by carbon-bearing solids. According to cosmological models of stellar evolution circumstellar dust accreted into protoplanets wherein thermal and/or aqueous processes will alter the dust under initially, non-equilibrium conditions.

Small-scale spatial variability in phylogenetic community structure during early plant succession depends on soil properties.

PubMed

Ulrich, Werner; Piwczyński, Marcin; Zaplata, Markus Klemens; Winter, Susanne; Schaaf, Wolfgang; Fischer, Anton

2014-07-01

During early plant succession, the phylogenetic structure of a community changes in response to important environmental filters and emerging species interactions. We traced the development of temperate-zone plant communities during the first 7 years of primary succession on catchment soils to explore patterns of initial species assembly. We found pronounced small-scale differences in the phylogenetic composition of neighbouring plant assemblages and a large-scale trend towards phylogenetic evenness. This small-scale variability appears to be mediated by soil properties, particularly carbonate content. Therefore, abiotic environmental conditions might counteract or even supersede the effects of interspecific competition among closely related species, which are usually predicted to exhibit patterns of phylogenetic evenness. We conclude that theories on phylogenetic community composition need to incorporate effects of small-scale variability of environmental factors.

Large- and small-scale constraints on power spectra in Omega = 1 universes

NASA Technical Reports Server (NTRS)

Gelb, James M.; Gradwohl, Ben-Ami; Frieman, Joshua A.

1993-01-01

The CDM model of structure formation, normalized on large scales, leads to excessive pairwise velocity dispersions on small scales. In an attempt to circumvent this problem, we study three scenarios (all with Omega = 1) with more large-scale and less small-scale power than the standard CDM model: (1) cold dark matter with significantly reduced small-scale power (inspired by models with an admixture of cold and hot dark matter); (2) cold dark matter with a non-scale-invariant power spectrum; and (3) cold dark matter with coupling of dark matter to a long-range vector field. When normalized to COBE on large scales, such models do lead to reduced velocities on small scales and they produce fewer halos compared with CDM. However, models with sufficiently low small-scale velocities apparently fail to produce an adequate number of halos.

Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud.

PubMed

Howk, J Christopher; Lehner, Nicolas; Fields, Brian D; Mathews, Grant J

2012-09-06

The primordial abundances of light elements produced in the standard theory of Big Bang nucleosynthesis (BBN) depend only on the cosmic ratio of baryons to photons, a quantity inferred from observations of the microwave background. The predicted primordial (7)Li abundance is four times that measured in the atmospheres of Galactic halo stars. This discrepancy could be caused by modification of surface lithium abundances during the stars' lifetimes or by physics beyond the Standard Model that affects early nucleosynthesis. The lithium abundance of low-metallicity gas provides an alternative constraint on the primordial abundance and cosmic evolution of lithium that is not susceptible to the in situ modifications that may affect stellar atmospheres. Here we report observations of interstellar (7)Li in the low-metallicity gas of the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun's metallicity. The present-day (7)Li abundance of the Small Magellanic Cloud is nearly equal to the BBN predictions, severely constraining the amount of possible subsequent enrichment of the gas by stellar and cosmic-ray nucleosynthesis. Our measurements can be reconciled with standard BBN with an extremely fine-tuned depletion of stellar Li with metallicity. They are also consistent with non-standard BBN.

Surveying Galaxy Evolution in the Far-Infrared: A Far-Infrared All-Sky Survey Concept

NASA Technical Reports Server (NTRS)

Benford, D. J.; Amato, M. J.; Dwek, E.; Freund, M. M.; Gardner, J. P.; Kashlinsky, A.; Leisawitz, D. T.; Mather, J. C.; Moseley, S. H.; Shafer, R. A.

2004-01-01

Half of the total luminosity in the Universe is emitted at rest wavelengths approximately 80-100 microns. At the highest known galaxy redshifts (z greater than or equal to 6) this energy is redshifted to approximately 600 microns. Quantifying the evolution of galaxies at these wavelengths is crucial to our understanding of the formation of structure in the Universe following the big bang. Surveying the whole sky will find the rare and unique objects, enabling follow-up observations. SIRCE, the Survey of Infrared Cosmic Evolution, is such a mission concept under study at NASA's Goddard Space Flight Center. A helium-cooled telescope with ultrasensitive detectors can image the whole sky to the confusion limit in 6 months. Multiple wavelength bands permit the extraction of photometric redshifts, while a large telescope yields a low confusion limit. We discuss the implications of such a survey for galaxy formation and evolution, large-scale structure, star formation, and the structure of interstellar dust.

A Universe without Weak Interactions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Harnik, Roni; Kribs, Graham D.; Perez, Gilad

2006-04-07

A universe without weak interactions is constructed that undergoes big-bang nucleosynthesis, matter domination, structure formation, and star formation. The stars in this universe are able to burn for billions of years, synthesize elements up to iron, and undergo supernova explosions, dispersing heavy elements into the interstellar medium. These definitive claims are supported by a detailed analysis where this hypothetical ''Weakless Universe'' is matched to our Universe by simultaneously adjusting Standard Model and cosmological parameters. For instance, chemistry and nuclear physics are essentially unchanged. The apparent habitability of the Weakless Universe suggests that the anthropic principle does not determine the scalemore » of electroweak breaking, or even require that it be smaller than the Planck scale, so long as technically natural parameters may be suitably adjusted. Whether the multi-parameter adjustment is realized or probable is dependent on the ultraviolet completion, such as the string landscape. Considering a similar analysis for the cosmological constant, however, we argue that no adjustments of other parameters are able to allow the cosmological constant to raise up even remotely close to the Planck scale while obtaining macroscopic structure. The fine-tuning problems associated with the electroweak breaking scale and the cosmological constant therefore appear to be qualitatively different from the perspective of obtaining a habitable universe.« less

Polarization of submillimetre lines from interstellar medium

NASA Astrophysics Data System (ADS)

Zhang, Heshou; Yan, Huirong

2018-04-01

Magnetic fields play important roles in many astrophysical processes. However, there is no universal diagnostic for the magnetic fields in the interstellar medium (ISM) and each magnetic tracer has its limitation. Any new detection method is thus valuable. Theoretical studies have shown that submillimetre fine-structure lines are polarized due to atomic alignment by ultraviolet photon-excitation, which opens up a new avenue to probe interstellar magnetic fields. We will, for the first time, perform synthetic observations on the simulated three-dimensional ISM to demonstrate the measurability of the polarization of submillimetre atomic lines. The maximum polarization for different absorption and emission lines expected from various sources, including star-forming regions are provided. Our results demonstrate that the polarization of submillimetre atomic lines is a powerful magnetic tracer and add great value to the observational studies of the submilimetre astronomy.

Application of pinch-and-swell structure rheology gauge to determine rock paleo-rheological parameters in Taili, western Liaoning, NE China

NASA Astrophysics Data System (ADS)

Sun, Zhengquan; Zeng, Zuoxun; Wu, Linbo; Xu, Shaopeng; Yang, Shuang; Chen, Deli; Wang, Jianxiu

2017-05-01

New results, in combination with previously published ones, reveal that when the Stress Exponent of the Competent layer (SEC) ranges from 1 to 10 (1 < n < 10), Pinch-and-Swell structure Rheology Gauge (PSRG) can only be available under the condition that the Viscosity ratio between the Competent layer and its corresponding Matrix layer (VCM) is larger than 10. Therefore, we made the attempt to calculate the viscosity ratio of pinch-and-swell structure of competent layer to the related matrix and stress exponent. Based on this knowledge, we applied this gauge to calculate SECs and VCMs of eight types of pinch-and-swell structures, which are widely developed in the Taili area of the west Liaoning Province in China. Statistical analysis of the SEC resulted in intervals of four types of competent layers, that is, Medium-scale Granitic coarse-to-pegmatitic Veins, Small-scale Augen Granite aplite Veins, Small-scale Granite aplite Veins, and Small-scale Augen Quartz-K-feldspar veins, with intervals of [3.50, 4.63], [2.64, 4.29], [2.70, 3.51], and [2.50, 3.36] respectively. The preferred intervals of VCM of the five types of pinch-and-swell structures, Small-scale Augen Granite aplite Veins + Fine-grained Biotite-Hornblende-plagioclase Gneiss, Medium-scale Granitic coarse-to-pegmatitic Veins + Fine-grained Biotite-Hornblende-plagioclase Gneiss, Small-scale Augen Granite aplite Veins + medium-to-fine-grained granitic gneiss, Medium-scale Granitic coarse-to-pegmatitic Veins + medium-to-fine-grained granitic gneiss, and Small-scale Augen Granite aplite Veins + fine-grained biotite-plagioclase gneiss, are [19.98, 62.51], [15.90, 61.17], [26.72, 93.27], [22.21, 107.26], and [76.33, 309.39] respectively. The similarities between these calculated SEC statistical preferred intervals and the physical experimental results verify the validity of the PSRG. The competent layers of the pinch-and-swell structures were presented in this study as power-law flow with SEC values that increased with the thickness of the layer. Grain-size plays an important role in the rheology of pinch-and-swell structures. The results offer a case for the application of PSRG and determine the key rock rheological parameters of North China Craton for future related studies.

Observations of O VI Emission from the Diffuse Interstellar Medium

NASA Technical Reports Server (NTRS)

Shelton, R. L.; Kruk, J. W.; Murphy, E. M.; Andersson, B. G.; Blair, W. P.; Dixon, W. V.; Edelstein, J.; Fullerton, A. W.; Gry, C.; Howk, J. C.;

Fullerenes in Space.

PubMed

Maier, John P; Campbell, Ewen K

2017-04-24

In 1985 the football structure of C 60 , buckminsterfullerene was proposed and subsequently confirmed following its macroscopic synthesis in 1990. From the very beginning the role of C 60 and C 60 + in space was considered, particularly in the context of the enigmatic diffuse interstellar bands. These are absorption features found in the spectra of reddened star light. The first astronomical observations were made around one hundred years ago and despite significant efforts none of the interstellar molecules responsible have been identified. The absorption spectrum of C 60 + was measured in a 5 K neon matrix in 1993 and two prominent bands near 9583 Å and 9645 Å were observed. On the basis of this data the likely wavelength range in which the gas phase C 60 + absorptions should lie was predicted. In 1994 two diffuse interstellar bands were found in this spectral region and proposed to be due to C 60 + . It took over 20 years to measure the absorption spectrum of C 60 + under conditions similar to those prevailing in diffuse clouds. In 2015, sophisticated laboratory experiments led to the confirmation that these two interstellar bands are indeed caused by C 60 + , providing the first answer to this century old puzzle. Here, we describe the experiments, concepts and astronomical observations that led to the detection of C 60 + in interstellar space. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

New ALMA constraints on the star-forming interstellar medium at low metallicity: a 50 pc view of the blue compact dwarf galaxy SBS 0335-052

NASA Astrophysics Data System (ADS)

Cormier, D.; Bendo, G. J.; Hony, S.; Lebouteiller, V.; Madden, S. C.; Galliano, F.; Glover, S. C. O.; Klessen, R. S.; Abel, N. P.; Bigiel, F.; Clark, P. C.

2017-06-01

Properties of the cold interstellar medium of low-metallicity galaxies are not well known due to the faintness and extremely small scale on which emission is expected. We present deep ALMA band 6 (230 GHz) observations of the nearby, low-metallicity (12 + log (O/H) = 7.25) blue compact dwarf galaxy SBS 0335-052 at an unprecedented resolution of 0.2 arcsec (52 pc). The 12CO J = 2→1 line is not detected and we report a 3σ upper limit of LCO(2-1) = 3.6 × 104 K km s-1 pc2. Assuming that molecular gas is converted into stars with a given depletion time, ranging from 0.02 to 2 Gyr, we find lower limits on the CO-to-H2 conversion factor αCO in the range 102-104 M⊙ pc-2 (K km s-1)-1. The continuum emission is detected and resolved over the two main super star clusters. Re-analysis of the IR-radio spectral energy distribution suggests that the mm-fluxes are not only free-free emission but are most likely also associated with a cold dust component coincident with the position of the brightest cluster. With standard dust properties, we estimate its mass to be as large as 105 M⊙. Both line and continuum results suggest the presence of a large cold gas reservoir unseen in CO even with ALMA.

Eyes in the sky. Interactions between asymptotic giant branch star winds and the interstellar magnetic field

NASA Astrophysics Data System (ADS)

van Marle, A. J.; Cox, N. L. J.; Decin, L.

2014-10-01

Context. The extended circumstellar envelopes (CSEs) of evolved low-mass stars display a large variety of morphologies. Understanding the various mechanisms that give rise to these extended structures is important to trace their mass-loss history. Aims: Here, we aim to examine the role of the interstellar magnetic field in shaping the extended morphologies of slow dusty winds of asymptotic giant branch (AGB) stars in an effort to pin-point the origin of so-called eye shaped CSEs of three carbon-rich AGB stars. In addition, we seek to understand if this pre-planetary nebula (PN) shaping can be responsible for asymmetries observed in PNe. Methods: Hydrodynamical simulations are used to study the effect of typical interstellar magnetic fields on the free-expanding spherical stellar winds as they sweep up the local interstellar medium (ISM). Results: The simulations show that typical Galactic interstellar magnetic fields of 5 to 10 μG are sufficient to alter the spherical expanding shells of AGB stars to appear as the characteristic eye shape revealed by far-infrared observations. The typical sizes of the simulated eyes are in accordance with the observed physical sizes. However, the eye shapes are transient in nature. Depending on the stellar and interstellar conditions, they develop after 20 000 to 200 000 yrs and last for about 50 000 to 500 000 yrs, assuming that the star is at rest relative to the local interstellar medium. Once formed, the eye shape develops lateral outflows parallel to the magnetic field. The explosion of a PN in the centre of the eye-shaped dust shell gives rise to an asymmetrical nebula with prominent inward pointing Rayleigh-Taylor instabilities. Conclusions: Interstellar magnetic fields can clearly affect the shaping of wind-ISM interaction shells. The occurrence of the eyes is most strongly influenced by stellar space motion and ISM density. Observability of this transient phase is favoured for lines-of-sight perpendicular to the interstellar magnetic field direction. The simulations indicate that shaping of the pre-PN envelope can strongly affect the shape and size of PNe. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Movies are available in electronic form at http://www.aanda.org

Dislocation dynamics simulations of plasticity at small scales

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhou, Caizhi

2010-01-01

As metallic structures and devices are being created on a dimension comparable to the length scales of the underlying dislocation microstructures, the mechanical properties of them change drastically. Since such small structures are increasingly common in modern technologies, there is an emergent need to understand the critical roles of elasticity, plasticity, and fracture in small structures. Dislocation dynamics (DD) simulations, in which the dislocations are the simulated entities, offer a way to extend length scales beyond those of atomistic simulations and the results from DD simulations can be directly compared with the micromechanical tests. The primary objective of this researchmore » is to use 3-D DD simulations to study the plastic deformation of nano- and micro-scale materials and understand the correlation between dislocation motion, interactions and the mechanical response. Specifically, to identify what critical events (i.e., dislocation multiplication, cross-slip, storage, nucleation, junction and dipole formation, pinning etc.) determine the deformation response and how these change from bulk behavior as the system decreases in size and correlate and improve our current knowledge of bulk plasticity with the knowledge gained from the direct observations of small-scale plasticity. Our simulation results on single crystal micropillars and polycrystalline thin films can march the experiment results well and capture the essential features in small-scale plasticity. Furthermore, several simple and accurate models have been developed following our simulation results and can reasonably predict the plastic behavior of small scale materials.« less

Definition of the large-scale extinction - A new solution of the central void phenomenon

NASA Astrophysics Data System (ADS)

Grabinska, T.

1989-11-01

The Estonian School results (Joeveer et al., 1977) concerning the galaxy cell structure were partly interpreted in the spirit of Zwicky's idea on intergalactic dust, although it remained a speculation. The same view was proposed also by Rudnicki to explain the evident deficit of galaxies in the central region (central void) of the Jagellonian field. Zieba (1974) tried to explain the effect of the central void in terms of an interstellar obscuration. This explanation of the central void (CV) in terms of a possible intergalactic dust was opened up again in 1984 and 1985; and its merits considered by Rudnicki et al. (1989). Now a new solution of the problem of the CV mystery is presented.

A TALE OF TWO MYSTERIES IN INTERSTELLAR ASTROPHYSICS: THE 2175 A EXTINCTION BUMP AND DIFFUSE INTERSTELLAR BANDS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xiang, F. Y.; Zhong, J. X.; Li Aigen, E-mail: jxzhong@xtu.edu.cn, E-mail: lia@missouri.edu

2011-06-01

The diffuse interstellar bands (DIBs) are ubiquitous absorption spectral features arising from the tenuous material in the space between stars-the interstellar medium (ISM). Since their first detection nearly nine decades ago, over 400 DIBs have been observed in the visible and near-infrared wavelength range in both the Milky Way and external galaxies, both nearby and distant. However, the identity of the species responsible for these bands remains as one of the most enigmatic mysteries in astrophysics. An equally mysterious interstellar spectral signature is the 2175 A extinction bump, the strongest absorption feature observed in the ISM. Its carrier also remainsmore » unclear since its first detection 46 years ago. Polycyclic aromatic hydrocarbon (PAH) molecules have long been proposed as a candidate for DIBs as their electronic transitions occur in the wavelength range where DIBs are often found. In recent years, the 2175 A extinction bump is also often attributed to the {pi}-{pi}* transition in PAHs. If PAHs are indeed responsible for both the 2175 A extinction feature and DIBs, their strengths may correlate. We perform an extensive literature search for lines of sight for which both the 2175 A extinction feature and DIBs have been measured. Unfortunately, we found no correlation between the strength of the 2175 A feature and the equivalent widths of the strongest DIBs. A possible explanation might be that DIBs are produced by small free gas-phase PAH molecules and ions, while the 2175 A bump is mainly from large PAHs or PAH clusters in condensed phase so that there is no tight correlation between DIBs and the 2175 A bump.« less

Three-dimensional Features of the Outer Heliosphere Due to Coupling between the Interstellar and Heliospheric Magnetic Field. V. The Bow Wave, Heliospheric Boundary Layer, Instabilities, and Magnetic Reconnection

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pogorelov, N. V.; Heerikhuisen, J.; Roytershteyn, V.

The heliosphere is formed due to interaction between the solar wind (SW) and local interstellar medium (LISM). The shape and position of the heliospheric boundary, the heliopause, in space depend on the parameters of interacting plasma flows. The interplay between the asymmetrizing effect of the interstellar magnetic field and charge exchange between ions and neutral atoms plays an important role in the SW–LISM interaction. By performing three-dimensional, MHD plasma/kinetic neutral atom simulations, we determine the width of the outer heliosheath—the LISM plasma region affected by the presence of the heliosphere—and analyze quantitatively the distributions in front of the heliopause. Itmore » is shown that charge exchange modifies the LISM plasma to such extent that the contribution of a shock transition to the total variation of plasma parameters becomes small even if the LISM velocity exceeds the fast magnetosonic speed in the unperturbed medium. By performing adaptive mesh refinement simulations, we show that a distinct boundary layer of decreased plasma density and enhanced magnetic field should be observed on the interstellar side of the heliopause. We show that this behavior is in agreement with the plasma oscillations of increasing frequency observed by the plasma wave instrument onboard Voyager 1. We also demonstrate that Voyager observations in the inner heliosheath between the heliospheric termination shock and the heliopause are consistent with dissipation of the heliospheric magnetic field. The choice of LISM parameters in this analysis is based on the simulations that fit observations of energetic neutral atoms performed by Interstellar Boundary Explorer .« less

Formation of globular cluster candidates in merging proto-galaxies at high redshift: a view from the FIRE cosmological simulations

DOE PAGES

Kim, Ji-hoon; Ma, Xiangcheng; Grudić, Michael Y.; ...

2017-11-23

Using a state-of-the-art cosmological simulation of merging proto-galaxies at high redshift from the FIRE project, with explicit treatments of star formation and stellar feedback in the interstellar medium, we investigate the formation of star clusters and examine one of the formation hypotheses of present-day metal-poor globular clusters. Here, we find that frequent mergers in high-redshift proto-galaxies could provide a fertile environment to produce long-lasting bound star clusters. The violent merger event disturbs the gravitational potential and pushes a large gas mass of ≳ 10 5–6 M ⊙ collectively to high density, at which point it rapidly turns into stars beforemore » stellar feedback can stop star formation. The high dynamic range of the reported simulation is critical in realizing such dense star-forming clouds with a small dynamical time-scale, tff ≲ 3 Myr, shorter than most stellar feedback time-scales. Our simulation then allows us to trace how clusters could become virialized and tightly bound to survive for up to ~420 Myr till the end of the simulation. Finally, because the cluster's tightly bound core was formed in one short burst, and the nearby older stars originally grouped with the cluster tend to be preferentially removed, at the end of the simulation the cluster has a small age spread.« less

Formation of globular cluster candidates in merging proto-galaxies at high redshift: a view from the FIRE cosmological simulations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Ji-hoon; Ma, Xiangcheng; Grudić, Michael Y.

Using a state-of-the-art cosmological simulation of merging proto-galaxies at high redshift from the FIRE project, with explicit treatments of star formation and stellar feedback in the interstellar medium, we investigate the formation of star clusters and examine one of the formation hypotheses of present-day metal-poor globular clusters. Here, we find that frequent mergers in high-redshift proto-galaxies could provide a fertile environment to produce long-lasting bound star clusters. The violent merger event disturbs the gravitational potential and pushes a large gas mass of ≳ 10 5–6 M ⊙ collectively to high density, at which point it rapidly turns into stars beforemore » stellar feedback can stop star formation. The high dynamic range of the reported simulation is critical in realizing such dense star-forming clouds with a small dynamical time-scale, tff ≲ 3 Myr, shorter than most stellar feedback time-scales. Our simulation then allows us to trace how clusters could become virialized and tightly bound to survive for up to ~420 Myr till the end of the simulation. Finally, because the cluster's tightly bound core was formed in one short burst, and the nearby older stars originally grouped with the cluster tend to be preferentially removed, at the end of the simulation the cluster has a small age spread.« less

Formation of globular cluster candidates in merging proto-galaxies at high redshift: a view from the FIRE cosmological simulations

NASA Astrophysics Data System (ADS)

Kim, Ji-hoon; Ma, Xiangcheng; Grudić, Michael Y.; Hopkins, Philip F.; Hayward, Christopher C.; Wetzel, Andrew; Faucher-Giguère, Claude-André; Kereš, Dušan; Garrison-Kimmel, Shea; Murray, Norman

2018-03-01

Using a state-of-the-art cosmological simulation of merging proto-galaxies at high redshift from the FIRE project, with explicit treatments of star formation and stellar feedback in the interstellar medium, we investigate the formation of star clusters and examine one of the formation hypotheses of present-day metal-poor globular clusters. We find that frequent mergers in high-redshift proto-galaxies could provide a fertile environment to produce long-lasting bound star clusters. The violent merger event disturbs the gravitational potential and pushes a large gas mass of ≳ 105-6 M⊙ collectively to high density, at which point it rapidly turns into stars before stellar feedback can stop star formation. The high dynamic range of the reported simulation is critical in realizing such dense star-forming clouds with a small dynamical time-scale, tff ≲ 3 Myr, shorter than most stellar feedback time-scales. Our simulation then allows us to trace how clusters could become virialized and tightly bound to survive for up to ˜420 Myr till the end of the simulation. Because the cluster's tightly bound core was formed in one short burst, and the nearby older stars originally grouped with the cluster tend to be preferentially removed, at the end of the simulation the cluster has a small age spread.

Abundance of live 244Pu in deep-sea reservoirs on Earth points to rarity of actinide nucleosynthesis

PubMed Central

Wallner, A.; Faestermann, T.; Feige, J.; Feldstein, C.; Knie, K.; Korschinek, G.; Kutschera, W.; Ofan, A.; Paul, M.; Quinto, F.; Rugel, G.; Steier, P.

2015-01-01

Half of the heavy elements including all actinides are produced in r-process nucleosynthesis, whose sites and history remain a mystery. If continuously produced, the Interstellar Medium is expected to build-up a quasi-steady state of abundances of short-lived nuclides (with half-lives ≤100 My), including actinides produced in r-process nucleosynthesis. Their existence in today’s interstellar medium would serve as a radioactive clock and would establish that their production was recent. In particular 244Pu, a radioactive actinide nuclide (half-life=81 My), can place strong constraints on recent r-process frequency and production yield. Here we report the detection of live interstellar 244Pu, archived in Earth’s deep-sea floor during the last 25 My, at abundances lower than expected from continuous production in the Galaxy by about 2 orders of magnitude. This large discrepancy may signal a rarity of actinide r-process nucleosynthesis sites, compatible with neutron-star mergers or with a small subset of actinide-producing supernovae. PMID:25601158

The analysis of ensembles of moderately saturated interstellar lines

NASA Technical Reports Server (NTRS)

Jenkins, E. B.

1986-01-01

It is shown that the combined equivalent widths for a large population of Gaussian-like interstellar line components, each with different central optical depths tau(0) and velocity dispersions b, exhibit a curve of growth (COG) which closely mimics that of a single, pure Gaussian distribution in velocity. Two parametric distributions functions for the line populations are considered: a bivariate Gaussian for tau(0) and b and a power law distribution for tau(0) combined with a Gaussian dispersion for b. First, COGs for populations having an extremely large number of nonoverlapping components are derived, and the implications are shown by focusing on the doublet-ratio analysis for a pair of lines whose f-values differ by a factor of two. The consequences of having, instead of an almost infinite number of lines, a relatively small collection of components added together for each member of a doublet are examined. The theory of how the equivalent widths grow for populations of overlapping Gaussian profiles is developed. Examples of the composite COG analysis applied to existing collections of high-resolution interstellar line data are presented.

The influence of Oort clouds on the mass and chemical balance of the interstellar medium

NASA Technical Reports Server (NTRS)

Stern, S. Alan; Shull, J. Michael

1990-01-01

The contribution of stellar encounters and interstellar erosion to comet cloud mass injection to the ISM is calculated. It is shown that evaporative mass loss from passing stars and SNe results in an average Galactic mass injection rate of up to 10 to the -5th solar mass/yr if such clouds are frequent around solar-type stars. Cometary erosion by interstellar grains produces an injection rate of 10 to the -5th to 10 to the -4th solar mass/yr. An injection rate of 2 x 10 to the -5th solar mass/yr is calculated. Each of these rates could be increased by a factor of about 15 if the comet clouds contain a significant amount of smaller debris. It is concluded that the total mass injection rate of material to the ISM by comet clouds is small compared to other ISM mass injection sources. Comet cloud mass loss to the ISM could be responsible for a sizeable fraction of the metal and dust abundances of the ISM if Oort clouds are common.

Reactivity of phosphorus mononitride and interstellar formation of molecules containing phospazo linkage: A computational study on the reaction between HSi (X2Γ) and PN (X1Σ+)

NASA Astrophysics Data System (ADS)

Bhasi, Priya; Nhlabatsi, Zanele P.; Sitha, Sanyasi

2017-12-01

Phosphorus mononitride (PN) shows some interesting chemistry due to its low dissociation energy (compared to N2) and small dipole moment (zero dipole moment for N2). In this work, a reaction between HSi (X2Γ) and PN (X1Σ+) has been studied using various computational methods. Analysis of the doublet surface of the HSi+PN reaction indicates that the reaction is exothermic in nature leading to the formation of various products. In view of the barrierless association of the reactants and exothermic nature for the product formation, it is suggested that species like HPNSi, cyclic-SiN(H)P (these two most stable isomers have phosphazo linkage) and HSiNP (third most stable isomer has phosphdiazo linkage) can possibly be detected in the interstellar medium. In view of the potential applications of phosphazo compounds in amide synthesis and pervasive nature of amide linkages in the nature, possible interstellar prebiotic applications can be advocated for these compounds.

Spitzer Observations of Dust Destruction in the Puppis A Supernova Remnant

NASA Technical Reports Server (NTRS)

Arendt, Richard G.; Dweek, Eli; Blair, William P.; Ghavamian, Parviz; Hwang, Una; Long, Knox X.; Petre, Robert; Rho, Jeonghee; Winkler, P. Frank

2010-01-01

The interaction of the Puppis A supernova remnant (SNR) with a neighboring molecular cloud provides a unique opportunity to measure the amount of grain destruction in an SNR shock. Spitzer Space Telescope MIPS imaging of the entire SNR at 24, 70, and 160 micrometers shows an extremely good correlation with X-ray emission, indicating that the SNR's IR radiation is dominated by the thermal emission of swept-up interstellar dust, collisionally heated by the hot shocked gas. Spitzer IRS spectral observations targeted both the Bright Eastern Knot (BEK) of the SNR where a small cloud has been engulfed by the supernova blast wave and outlying portions of the associated molecular cloud that are yet to be hit by the shock front. Modeling the spectra from both regions reveals the composition and the grain size distribution of the interstellar dust, both in front of and behind the SNR shock front. The comparison shows that the ubiquitous polycyclic aromatic hydrocarbons of the interstellar medium are destroyed within the BEK, along with nearly 25% of the mass of graphite and silicate dust grains.

Searching for Galactic hidden gas through interstellar scintillation: results from a test with the NTT-SOFI detector

NASA Astrophysics Data System (ADS)

Habibi, F.; Moniez, M.; Ansari, R.; Rahvar, S.

2011-01-01

Aims: Stars twinkle because their light propagates through the atmosphere. The same phenomenon is expected at a longer time scale when the light of remote stars crosses an interstellar molecular cloud, but it has never been observed at optical wavelength. In a favorable case, the light of a background star can be subject to stochastic fluctuations on the order of a few percent at a characteristic time scale of a few minutes. Our ultimate aim is to discover or exclude these scintillation effects to estimate the contribution of molecular hydrogen to the Galactic baryonic hidden mass. This feasibility study is a pathfinder toward an observational strategy to search for scintillation, probing the sensitivity of future surveys and estimating the background level. Methods: We searched for scintillation induced by molecular gas in visible dark nebulae as well as by hypothetical halo clumpuscules of cool molecular hydrogen (H2-He) during two nights. We took long series of 10 s infrared exposures with the ESO-NTT telescope toward stellar populations located behind visible nebulae and toward the Small Magellanic Cloud (SMC). We therefore searched for stars exhibiting stochastic flux variations similar to what is expected from the scintillation effect. According to our simulations of the scintillation process, this search should allow one to detect (stochastic) transverse gradients of column density in cool Galactic molecular clouds of order of ~ 3 × 10-5 g/cm2/10 000 km. Results: We found one light-curve that is compatible with a strong scintillation effect through a turbulent structure characterized by a diffusion radius Rdiff < 100 km in the B68 nebula. Complementary observations are needed to clarify the status of this candidate, and no firm conclusion can be established from this single observation. We can also infer limits on the existence of turbulent dense cores (of number density n > 109 cm-3) within the dark nebulae. Because no candidate is found toward the SMC, we are also able to establish upper limits on the contribution of gas clumpuscules to the Galactic halo mass. Conclusions: The limits set by this test do not seriously constrain the known models, but we show that the short time-scale monitoring for a few 106 star × hour in the visible band with a >4 m telescope and a fast readout camera should allow one to quantify the contribution of turbulent molecular gas to the Galactic halo. The LSST (Large Synoptic Survey Telescope) is perfectly suited for this search. This work is based on observations made at the European Southern Observatory, La Silla, Chile.

Multi-scale structures of turbulent magnetic reconnection

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nakamura, T. K. M., E-mail: takuma.nakamura@oeaw.ac.at; Nakamura, R.; Narita, Y.

2016-05-15

We have analyzed data from a series of 3D fully kinetic simulations of turbulent magnetic reconnection with a guide field. A new concept of the guide filed reconnection process has recently been proposed, in which the secondary tearing instability and the resulting formation of oblique, small scale flux ropes largely disturb the structure of the primary reconnection layer and lead to 3D turbulent features [W. Daughton et al., Nat. Phys. 7, 539 (2011)]. In this paper, we further investigate the multi-scale physics in this turbulent, guide field reconnection process by introducing a wave number band-pass filter (k-BPF) technique in whichmore » modes for the small scale (less than ion scale) fluctuations and the background large scale (more than ion scale) variations are separately reconstructed from the wave number domain to the spatial domain in the inverse Fourier transform process. Combining with the Fourier based analyses in the wave number domain, we successfully identify spatial and temporal development of the multi-scale structures in the turbulent reconnection process. When considering a strong guide field, the small scale tearing mode and the resulting flux ropes develop over a specific range of oblique angles mainly along the edge of the primary ion scale flux ropes and reconnection separatrix. The rapid merging of these small scale modes leads to a smooth energy spectrum connecting ion and electron scales. When the guide field is sufficiently weak, the background current sheet is strongly kinked and oblique angles for the small scale modes are widely scattered at the kinked regions. Similar approaches handling both the wave number and spatial domains will be applicable to the data from multipoint, high-resolution spacecraft observations such as the NASA magnetospheric multiscale (MMS) mission.« less

Multi-scale structures of turbulent magnetic reconnection

NASA Astrophysics Data System (ADS)

Nakamura, T. K. M.; Nakamura, R.; Narita, Y.; Baumjohann, W.; Daughton, W.

2016-05-01

We have analyzed data from a series of 3D fully kinetic simulations of turbulent magnetic reconnection with a guide field. A new concept of the guide filed reconnection process has recently been proposed, in which the secondary tearing instability and the resulting formation of oblique, small scale flux ropes largely disturb the structure of the primary reconnection layer and lead to 3D turbulent features [W. Daughton et al., Nat. Phys. 7, 539 (2011)]. In this paper, we further investigate the multi-scale physics in this turbulent, guide field reconnection process by introducing a wave number band-pass filter (k-BPF) technique in which modes for the small scale (less than ion scale) fluctuations and the background large scale (more than ion scale) variations are separately reconstructed from the wave number domain to the spatial domain in the inverse Fourier transform process. Combining with the Fourier based analyses in the wave number domain, we successfully identify spatial and temporal development of the multi-scale structures in the turbulent reconnection process. When considering a strong guide field, the small scale tearing mode and the resulting flux ropes develop over a specific range of oblique angles mainly along the edge of the primary ion scale flux ropes and reconnection separatrix. The rapid merging of these small scale modes leads to a smooth energy spectrum connecting ion and electron scales. When the guide field is sufficiently weak, the background current sheet is strongly kinked and oblique angles for the small scale modes are widely scattered at the kinked regions. Similar approaches handling both the wave number and spatial domains will be applicable to the data from multipoint, high-resolution spacecraft observations such as the NASA magnetospheric multiscale (MMS) mission.

Multi-phase Nature of a Radiation-driven Fountain with Nuclear Starburst in a Low-mass Active Galactic Nucleus

NASA Astrophysics Data System (ADS)

Wada, Keiichi; Schartmann, Marc; Meijerink, Rowin

2016-09-01

The structures and dynamics of molecular, atomic, and ionized gases are studied around a low-luminosity active galactic nucleus (AGN) with a small (2× {10}6{M}⊙ ) black hole using three-dimensional (3D) radiation-hydrodynamic simulations. We studied, for the first time, the non-equilibrium chemistry for the X-ray-dominated region in the “radiation-driven fountain” with supernova feedback. A double hollow cone structure is naturally formed without postulating a thick “torus” around a central source. The cone is occupied with an inhomogeneous, diffuse ionized gas and surrounded by a geometrically thick (h/r≳ 1) atomic gas. Dense molecular gases are distributed near the equatorial plane, and energy feedback from supernovae enhances their scale height. Molecular hydrogen exists in a hot phase (>1000 K) as well as in a cold (\\lt 100 {{K}}), dense (\\gt {10}3 {{cm}}-3) phase. The velocity dispersion of H2 in the vertical direction is comparable to the rotational velocity, which is consistent with near-infrared observations of nearby Seyfert galaxies. Using 3D radiation transfer calculations for the dust emission, we find polar emission in the mid-infrared band (12 μm), which is associated with bipolar outflows, as suggested in recent interferometric observations of nearby AGNs. If the viewing angle for the nucleus is larger than 75°, the spectral energy distribution is consistent with that of the Circinus galaxy. The multi-phase interstellar medium observed in optical/infrared and X-ray observations is also discussed.

Astrochemistry: Recent Advances in the Study of Carbon Molecules in Space

NASA Technical Reports Server (NTRS)

Salama, Farid

2006-01-01

Carbon molecules and ions play an important role in space. Polycyclic Aromatic Hydrocarbons (PAHs) are the best-known candidates to account for the infrared emission bands (UIR bands) and PAH spectral features are now being used as probes of the interstellar medium in Galactic and extra-galactic environments. PAHs are also thought to be among the carriers of the diffuse interstellar absorption bands (DIBs). In the model dealing with the interstellar spectral features, PAHs are present as a mixture of radicals, ions and neutral species. PAH ionization states reflect the ionization balance of the medium while PAH size, composition, and structure reflect the energetic and chemical history of the medium. A major challenge for laboratory Astrochemistry is to reproduce (in a realistic way) the physical conditions that exist in the emission and absorption interstellar zones. An extensive laboratory program has been developed in various laboratories to characterize the physical and chemical properties of PAHs in astrophysical environments and to describe how they influence the radiation and energy balance in space and the interstellar chemistry. In particular, laboratory experiments provide measurements of the spectral characteristics of interstellar PAH analogs from the ultraviolet and visible range to the infrared range for comparison with astronomical data. The harsh physical conditions of the interstellar medium - characterized by a low temperature, an absence of collisions and strong ultraviolet radiation fields - are simulated in the laboratory by associating a molecular beam with an ionizing discharge to generate a cold plasma expansion. PAH ions are formed from the neutral precursors in an isolated environment at low temperature (of the order of 100 K). The spectra of neutral and ionized PAHs are measured using the high sensitivity methods of cavity ring down spectroscopy (CRDS). These experiments provide unique information on the spectra of free, cold large carbon molecules and ions in the gas phase.

Star formation and abundances in the nearby irregular galaxy VII ZW 403

NASA Astrophysics Data System (ADS)

Tully, R. B.; Boesgaard, A. M.; Dyck, H. M.; Schempp, W. V.

1981-05-01

Photometry in J, H, and K bands reveals that there is an unresolved source of infrared emission associated with the brightest H II region in VII Zw 403, and the colors suggest the presence of a substantial number of K and M supergiants in addition to the hot O stars that must be present to account for the ionized gas. Spectrophotometry of this emission region indicates that reddening is substantial, and that the interpretation of the observed Balmer decrement in terms of reddening is not straightforward. The primary nucleosynthesis products O, S, and Ne are underabundant compared with the sun by a factor of 15; N is underabundant compared with the sun by a factor of 160; and the helium abundance suggests that either there could have been only a small number of star formation episodes or the galaxy is younger than the time scale of the process that deposits N in the interstellar medium.

Star formation across cosmic time and its influence on galactic dynamics

NASA Astrophysics Data System (ADS)

Freundlich, Jonathan

2015-12-01

Observations show that ten billion years ago, galaxies formed their stars at rates up to twenty times higher than now. As stars are formed from cold molecular gas, a high star formation rate means a significant gas supply, and galaxies near the peak epoch of star formation are indeed much more gas-rich than nearby galaxies. Is the decline of the star formation rate mostly driven by the diminishing cold gas reservoir, or are the star formation processes also qualitatively different earlier in the history of the Universe? Ten billion years ago, young galaxies were clumpy and prone to violent gravitational instabilities, which may have contributed to their high star formation rate. Stars indeed form within giant, gravitationally-bound molecular clouds. But the earliest phases of star formation are still poorly understood. Some scenarii suggest the importance of interstellar filamentary structures as a first step towards core and star formation. How would their filamentary geometry affect pre-stellar cores? Feedback mechanisms related to stellar evolution also play an important role in regulating star formation, for example through powerful stellar winds and supernovae explosions which expel some of the gas and can even disturb the dark matter distribution in which each galaxy is assumed to be embedded. This PhD work focuses on three perspectives: (i) star formation near the peak epoch of star formation as seen from observations at sub-galactic scales; (ii) the formation of pre-stellar cores within the filamentary structures of the interstellar medium; and (iii) the effect of feedback processes resulting from star formation and evolution on the dark matter distribution.

The hot interstellar medium in NGC 1399

NASA Technical Reports Server (NTRS)

Loewenstein, Michael; Serlemitsos, Peter J.

1993-01-01

The first two high signal-to-noise, broad bandpass x-ray spectra of elliptical galaxies were obtained with the Broad Band X-ray Telescope (BBXRT) as part of the December 1990 Astro mission. These observations provided unprecedented information on the thermal and metallicity structure of the hot interstellar media in two ellipticals: NGC 1399, the central galaxy in the Fornax cluster, and NGC 4472, the brightest galaxy in the Virgo cluster. The finalized analysis and interpretation of the approximately 4000 sec of BBXRT data on NGC 1399 is reported.

3D simulation of LISM oxygen flux with PUIs inside of heliosphere

DOE PAGES

Kawamura, Akito D.; Heerikhuisen, Jacob; Pogorelov, Nikolai V.; ...

2012-11-20

The structure of the heliospheric interface has attracted increasing attention with continual improvements in modelling and observations, during the last half decade. The Interstellar Boundary Explore (IBEX) spacecraft is returning important data that require a theoretical model of Heliosphere to ensure proper interpretation. Furthermore, we develop a framework for understanding the measurements of heavier-than-hydrogen atoms by IBEX in terms of a 3D MHD-neutral numerical solution of the sun's interaction with the interstellar medium, combined with a test particle approach for heavy atoms and ions.

Feedback by massive stars and the emergence of superbubbles. I. Energy efficiency and Vishniac instabilities

NASA Astrophysics Data System (ADS)

Krause, M.; Fierlinger, K.; Diehl, R.; Burkert, A.; Voss, R.; Ziegler, U.

2013-02-01

Context. Massive stars influence their environment through stellar winds, ionising radiation, and supernova explosions. This is signified by observed interstellar bubbles. Such feedback is an important factor for galaxy evolution theory and galactic wind models. The efficiency of the energy injection into the interstellar medium (ISM) via bubbles and superbubbles is uncertain, and is usually treated as a free parameter for galaxy scale effects. In particular, since many stars are born in groups, it is interesting to study the dependence of the effective energy injection on the concentration of the stars. Aims: We aim to reproduce observations of superbubbles, their relation to the energy injection of the parent stars, and to understand their effective energy input into the ISM, as a function of the spatial configuration of the group of parent stars. Methods: We study the evolution of isolated and merging interstellar bubbles of three stars (25, 32, and 60 M⊙) in a homogeneous background medium with a density of 10mp cm-3 via 3D-hydrodynamic simulations with standard ISM thermodynamics (optically thin radiative cooling and photo-electric heating) and time-dependent energy and mass input according to stellar evolutionary tracks. We vary the position of the three stars relative to each other to compare the energy response for cases of isolated, merging and initially cospatial bubbles. Results: Mainly due to the Vishniac instability, our simulated bubbles develop thick shells and filamentary internal structures in column density. The shell widths reach tens of per cent of the outer bubble radius, which compares favourably to observations. More energy is retained in the ISM for more closely packed groups, by up to a factor of three and typically a factor of two for intermediate times after the first supernova. Once the superbubble is established, different positions of the contained stars make only a minor difference to the energy tracks. For our case of three massive stars, the energy deposition varies only very little for distances up to about 30 pc between the stars. Energy injected by supernovae is entirely dissipated in a superbubble on a timescale of about 1 Myr, which increases slightly with the superbubble size at the time of the explosion. Conclusions: The Vishniac instability may be responsible for the broadening of the shells of interstellar bubbles. Massive star winds are significant energetically due to their - in the long run - more efficient, steady energy injection and because they evacuate the space around the massive stars. For larger scale simulations, the feedback effect of close groups of stars or clusters may be subsumed into one effective energy input with insignificant loss of energy accuracy. The movie associated to Fig. 3 is available at http://www.aanda.org

Recent observations of interstellar molecules - Detection of CCO and a limit on H2C3O

NASA Technical Reports Server (NTRS)

Brown, R. D.; Cragg, D. M.; Godfrey, P. D.; Irvine, W. M.; Mcgonagle, D.; Ohishi, M.

1992-01-01

In order to test gas-phase reaction schemes for the production of small oxides of carbon in cold, dense interstellar clouds, we have searched for the radical CCO and for propadienone (H2C3O) in Taurus Molecular Cloud 1, a nearby cloud which exhibits a rich organic chemistry. The radical CCO has been detected with a fractional abundance some two orders of magnitude less than that of CCS, about one order of magnitude less than that of H2CCO, and slightly less than that of C3O. An upper limit has been obtained on the abundance of propadienone which is slightly less than that of its isomer propynal (HC2CHO).

Impacto ambiental de los remanentes de supernova

NASA Astrophysics Data System (ADS)

Dubner, G. M.

2015-08-01

The explosion of a supernovae (SN) represents the sudden injection of about ergs of thermal and mechanical energy in a small region of space, causing the formation of powerful shock waves that propagate through the interstellar medium at speeds of several thousands of km/s. These waves sweep, compress and heat the interstellar material that they encounter, forming the supernova remnants. Their evolution over thousands of years change forever, irreversibly, not only the physical but also the chemical properties of a vast region of space that can span hundreds of parsecs. This contribution briefly analyzes the impact of these explosions, discussing the relevance of some phenomena usually associated with SNe and their remnants in the light of recent theoretical and observational results.

Element abundances at high redshift

NASA Technical Reports Server (NTRS)

Meyer, David M.; Welty, D. E.; York, D. G.

1989-01-01

Abundances of Si(+), S(+), Cr(+), Mn(+), Fe(_), and Zn(+) are considered for two absorption-line systems in the spectrum of the QSO PKS 0528 - 250. Zinc and sulfur are underabundant, relative to H, by a factor of 10 compared to their solar and Galactic interstellar abundances. The silicon-, chromium-, iron-, and nickel-to-hydrogen ratios are less than the solar values and comparable to the local interstellar ratios. A straightforward interpretation is that nucleosynthesis in these high-redshift systems has led to only about one-tenth as much heavy production as in the gas clouds around the sun, and that the amount of the observed underabundances attributable to grain depletion is small. The dust-to-gas ratio in these clouds is less than 8 percent of the Galactic value.

Parsec-scale jets and tori in seyfert galaxies

NASA Astrophysics Data System (ADS)

Roy, A. L.; Wrobel, J. M.; Wilson, A. S.; Ulvestad, J. S.; Norris, R. P.; Mundell, C. G.; Krichbaum, T. P.; Falcke, H.; Colbert, E. J. M.

2001-01-01

H. Falcke, T.P. Krichbaum, C.G. Mundell, J.S. Ulvestad, A.S. Wilson, J.M. Wrobel Active galaxies tend to be powerful or weak radio sources, and we still do not understand the underlying cause. Perhaps the engine is the same in both systems and the jet gets disrupted by dense interstellar medium in radio-quiet objects, or else the difference is intrinsic with jet power scaling with black hole spin. To distinguish, one can look for signs of interaction between the jet and the narrow-line region, and to measure the jet speed close to the jet base, before environmental effects become important. We find one-sided parsec-scale jet structures in Mrk 348, Mrk 231, NGC 4151, and NGC 5506 using VLBI, and we measure low jet speeds (typically <= 0.25 c). The one-sidedness probably is not due to Doppler boosting, but rather is probably free-free absorption. Plasma conditions required to produce the absorption are Ne >= 2 × 105 cm-3 assuming a path length of 0.1 pc, typical of that expected at the inner edge of the obscuring torus.

Energetic neutral atom and interstellar flow observations with IBEX: Implications for the global heliosphere

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schwadron, N. A., E-mail: nschwadron@unh.edu; Southwest Research Institute, San Antonio, TX, 78238; McComas, D. J.

2016-03-25

Since launch in Oct. 2008, IBEX, with its two energetic neutral atom (ENA) cameras, has provided humankind with the first-ever global images of the complex boundary separating the heliosphere from the local interstellar medium (LISM). IBEX’s energy-resolved all-sky maps, collected every six months, are yielding remarkable new insights into the heliospheres structure as it is shaped by the combined forces of the local interstellar flow, the local interstellar magnetic field (LISMF), and the evolving solar wind. IBEX has also acquired the first images of ENAs backscattered from the surface of the moon as well as global images of the magnetosphericmore » response to solar wind disturbances. IBEX thus addresses all three Heliophysics science objectives set forth in the 2014 Science Plan for NASAs Science Mission Directorate (SMD) as well as the goals in the recent Solar and Space Physics Decadal Survey (NRC 2012). In addition, with the information it provides on the properties of the LISM and the LISMF, IBEX represents a unique bridge between heliophysics and astrophysics, and fills in critical knowledge for understanding the habitability of exoplanetary systems and the future habitability of Earth and the solar system. Because of the few-year time lag due to solar wind and ENA transport, IBEX observed the solar wind/ LISM interaction characteristic of declining phase/solar minimum conditions. In the continuing mission, IBEX captures the response of the interstellar boundaries to the changing structure of the solar wind in its transition toward the “mini” solar maximum and possibly the decline into the next solar minimum. The continuing IBEX mission affords never-to-be-repeated opportunities to coordinate global imaging of the heliospheric boundary with in-situ measurements by the Voyagers as they pass beyond the heliopause and start to directly sample the LISM.« less